[moetemp]

What changed?
This PR adds the Stream service messages, marked experimental.

• stream/v1/message.proto has the shared types, streamservice/v1/request_response.proto has the requests and responses, and enums/v1/stream.proto has the enums.
• StreamRef.owner is a common.v1.Execution, the Workflow or Standalone Activity that owns the Stream.
• A read gives StreamRecord items. Each holds one oneof item: a StreamData Record, a Supersession, or the StreamClose. offset stays outside the oneof.
• CreateStream is create-or-get for an open Stream with the same owner. That Stream keeps its Retention, and created is false. A create of a closed Stream fails with CLOSED, of an expired one with EXPIRED, and of another owner's with NOT_OWNER. So CreateStreamResponse has no status.
• A create with an owner needs a ref whose owner names the owner's run chain, and the response's ref has that owner.
• A Stream carries one payload type, so a Record has no Topic and a read has no filter.
• The Topic fields of earlier drafts are reserved, since the SDK previews used them. So are auto_close (CreateStreamRequest field 5, DescribeStreamResponse field 4) and CreateStreamResponse.status (field 3).
• StreamErrorDetails.reason is the StreamErrorReason enum. Each case names its gRPC status code, so each SDK can map a reason to one error type.
• OFFSET is OUT_OF_RANGE. It's a read past the newest offset + 1, for example after a failover lost the newest Records, and a read from that offset works again later. A negative offset is INVALID.
• The comments describe no backend. An offset is ordered, unique and sparse, with no Redis entry id.
• No service.proto yet. It comes with the later stage of the project (Jetty).

Why?
The SDKs and Redis already use these messages, so they need one home before a server serves them.

Breaking changes
None. These are new files.

Server PR
None needed for now.