SNOW-1491199 Introduce ast.proto to the Snowpark client

[sfc-gh-oplaton]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-1491199

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines

3. Please describe how your code solves the related issue.

Introduce ast.proto under src/snowflake/snowpark/_internal/proto and ast_pb2.py under the generated subdirectory.
ast.proto gets created from the contents of Snowpark/ast, using ir-dsl-c. The chain is AST IR -> ir-dsl-c -> ast.proto -> Python protoc -> ast_pb2.py.
We are merging both ast.proto and ast_pb2.py because the Snowpark repository is not fully equipped to generate the Python client stubs at packaging time. This change is a work in progress and blocked behind IT tickets for an indeterminate amount of time.
A complete description of the AST model is too large to include here. We are currently working on an AST IR developer guide, as well.

[sfc-gh-lspiegelberg]

lgtm