api: align volume and container message/field names across ateapi.proto, atelet.proto, and ateom.proto #2419

internal/proto/ateletpb/atelet.proto and internal/proto/ateompb/ateom.proto use several different names for the same volume and container concepts defined in pkg/proto/ateapipb/ateapi.proto:

1. EnvVar vs EnvEntry (atelet.proto:405):
   ateapi.proto names a container environment variable EnvVar, whereas atelet.proto names the identical { name, value } message EnvEntry.
2. *Volume vs *VolumeSource (atelet.proto:293,349):
   In atelet.proto, Volume.source mixes ExternalVolumeSource and ImageVolumeSource with DurableDirVolume and SystemInfoVolume (whereas ateapi.proto uses DurableDirVolumeSource and SystemInfoVolumeSource).
3. external vs csi / bare volumes (ateom.proto:127,194):
   • ateom.Container names external volume mounts csi_volume_mounts (repeated VolumeMount csi_volume_mounts = 4) alongside durable_dir_volume_mounts, system_info_volume_mounts, and image_volume_mounts (even though external volumes can be backed by internal volume plugins as well as CSI drivers, and ateapi/atelet call them external).
   • ateom.ActorDirs names the external volume mount directory volumes_dir (ateom.proto:127) alongside durable_dir_volume_mounts_dir and system_info_volume_roots_dir.
4. Duplicate *VolumeMount messages and volume_name vs name (ateom.proto:234-268):
   ateom.proto defines four separate messages (VolumeMount, DurableDirVolumeMount, SystemInfoVolumeMount, ImageVolumeMount) that all have the exact same { string volume_name = 1; string mount_path = 2; } shape, and uses volume_name whereas ateapi.VolumeMount
   and atelet.VolumeMount use name.

Proposal

1. In internal/proto/ateletpb/atelet.proto:
   • Rename EnvEntry -> EnvVar.
   • Rename DurableDirVolume -> DurableDirVolumeSource and SystemInfoVolume -> SystemInfoVolumeSource.
2. In internal/proto/ateompb/ateom.proto:
   • Rename Container.csi_volume_mounts -> external_volume_mounts.
   • Rename ActorDirs.volumes_dir -> external_volume_mounts_dir.
   • Consolidate VolumeMount, DurableDirVolumeMount, SystemInfoVolumeMount, and ImageVolumeMount into a single VolumeMount { string name = 1; string mount_path = 2; } message (matching ateapi.VolumeMount and atelet.VolumeMount).

Benjamin Elder (@BenTheElder)