Support contentEncoding for OpenAPI 3.1 schemas (parse + ContentEncodingIn30 rule)

[takayamaki]

Continuing the OpenAPI 3.1 work from #152.

OpenAPI 3.1 adopts JSON Schema 2020-12, which brings the contentEncoding annotation.
It records how a string carrying binary data is encoded (base64, base32, quoted-printable, ...).
3.0 has no keyword for this.

This PR adds parse support and version-mismatch detection.
It is the sibling of #204 (contentMediaType).

Parse layer

contentEncoding is parsed as a plain value and exposed as an accessor on Schema:

schema.contentEncoding # => "base64"

Following the permissive-parse strategy agreed in #152,
the parse layer accepts contentEncoding regardless of the declared OpenAPI version.

contentEncoding is an annotation: it describes the payload rather than constraining it,
so there is no runtime validation to add here.

SpecValidator rule

ContentEncodingIn30 reports a violation for each schema in a 3.0 document that uses contentEncoding:

OpenAPIParser.load(
  'spec.yaml',
  strict_specification_version: :warn,
)
# [ContentEncodingIn30] #/components/schemas/Attachment — `contentEncoding` is a 3.1 addition (from JSON Schema 2020-12); 3.0 has no equivalent

Detection inspects raw_schema key presence,
so it fires on any declared encoding value.

[geemus]

lgtm, thanks!