[SPARK-56650][SDP][CONNECT] Add AutoCDC Spark Connect APIs

### What changes were proposed in this pull request?
Add the spark-connect protos to support AutoCDC flows. This is mainly two new protos:

`AutoCdcFlowDetails`: similar to the existing `WriteRelationFlowDetails`, describes an AutoCDC flow.
 `SCDType` an enum to specify the type of SCD (slow changing dimension) storage type.

For now, throw "not implemented" for the new flow type.

### Why are the changes needed?
This new API is needed to specify an AutoCDC flow in a declarative pipeline.

### Does this PR introduce _any_ user-facing change?
It introduces a new Spark Connect API, however, no implementation yet, and also no language wrappers (e.g. Python), yet.

### How was this patch tested?
No tests, as this API is unimplemented for now.

### Was this patch authored or co-authored using generative AI tooling?
Generated-by: Claude Sonnet 4.6

Closes #55589 from anew/add-sc-apis.

Authored-by: Andreas Neumann <anew@apache.org>
Signed-off-by: DB Tsai <dbtsai@dbtsai.com>
(cherry picked from commit a88c275)
Signed-off-by: DB Tsai <dbtsai@dbtsai.com>

Author: anew