Add support to configure connection options

[koletzilla]

Summary

Before this change, the driver only accepted a fixed list of options. There was no way to set
general ClickHouse settings like mutations_sync, alter_sync or insert_quorum, which
clients often need to pin for correctness.

This PR adds two ways to pass any ClickHouse setting:

1. A passthrough option key:

clickhouse.setting.<setting_name> = <value>

It can be set on ClickhouseDatabase, ClickhouseConnection and ClickhouseStatement. The
value propagates to objects created lower in the hierarchy, and lower levels can override it —
the same model as clickhouse.client.product_info. Values can be read back with
get_option_string().

2. Query parameters in the connection URI:

http://localhost:8123?mutations_sync=3

Every URI query parameter (except the existing protocol pseudo-parameter) is treated as a
setting, exactly as if it were set with the option key above. Previously these parameters were
silently dropped. This helps ADBC consumers that can only pass a URI string.

Internally both map directly to the clickhouse-rs client: Client::set_setting(name, value).
The client then sends the setting as a URL query parameter on every request, which is the native
way to carry "connection settings" over the ClickHouse HTTP interface (there is no server-side
connection state).

Usage example

let mut driver = ClickhouseDriver::init();

// Applies to every connection and statement created from this Database.
let db = driver.new_database_with_opts([
    (OptionDatabase::Uri, "http://localhost:8123/?insert_quorum=auto".into()),
    ("clickhouse.setting.mutations_sync".into(), "3".into()),
])?;

let mut conn = db.new_connection()?;

// Settings can also be set or changed on a live connection o
conn.set_option("clickhouse.setting.alter_sync".into(), "3".into())?;

// Read-back:
assert_eq!(conn.get_option_string("clickhouse.setting.mutations_sync".into())?, "3");

Decisions taken

• Why a key prefix? ADBC only offers flat SetOption(key, value) string pairs; there is no
  "pass a map" call. Encoding the setting name in a namespaced key is the standard ADBC pattern
  for open-ended option families (e.g. Flight SQL's adbc.flight.sql.rpc.call_header.<name>).
• String values only. Same as every existing option in this driver, and it matches how
  settings travel anyway (text query parameters). Int/double support can be added later without
  breaking anything.
• Database level stores a map. ClickhouseDatabase holds no client, so its settings are
  stored and replayed when a connection is created, before the per-connection options are
  applied, which is what makes lower levels win.
• URI parameters and options share that map, so precedence is simply "last write wins per
  setting", in whichever order they are set. A protocol parameter on an http(s):// URL is
  now rejected with InvalidArguments (it is only meaningful for clickhouse:// rewriting;
  before, it was silently carried and dropped).
• Small behavior change: URI query parameters used to be preserved in the stored URL (where
  clickhouse-rs dropped them at request time, so they never worked). Now they are folded into
  the settings map instead; test_set_uri expectations were updated.
• Read-back of an unset setting returns NotFound; an empty setting name
  ("clickhouse.setting.") is rejected with InvalidArguments.
• Overlap with clickhouse.client.* options is allowed. clickhouse.setting.session_id
  writes the same client state as clickhouse.client.session_id; last write wins. Note that
  query_id is regenerated for every new statement, so setting it higher up has no effect.
  This is documented on SETTING_PREFIX.
• ClickhouseConnection::get_option_string() is now implemented (it was blanket
  NotImplemented). This also fixes an existing gap: the SESSION_ID docs promised read-back
  that was never implemented.
• Prefix keys are handled with a strip_prefix early return instead of a match guard, because
  if-let match guards are not stable on the pinned Rust 1.91.0 toolchain.

Note for #67 (default database)

?database=x in the URI now folds like any other parameter and works through the HTTP
interface's database query parameter. If a dedicated default-database feature (#67) lands
later, it should take ownership of the database parameter s

Possible future changes

• Route the dedicated clickhouse.client.* options through the same internal path. They already share the same underlying client state; the per-key match arms could become a small alias table. Kept out of this PR to stay minimal.

[abonander]

Your test fix is merged so this just needs a rebase.

[koletzilla]

Rebase done!