Add Iceberg tag time travel support

[sfc-gh-igarish]

Add support for reading Iceberg tables using snapshot tags. This enables time travel queries using Iceberg snapshot tags with the AT(ICEBERG_TAG) syntax.

Changes:

• Add iceberg_tag field to TimeTravelConfig NamedTuple
• Update validate_and_normalize_params to handle iceberg_tag (only works with time_travel_mode='at')
• Update generate_sql_clause to produce AT(ICEBERG_TAG => 'tag_name')
• Add iceberg_tag parameter to Table.init, Session.table(), and DataFrameReader.table()
• Add TAG and ICEBERG_TAG options mapping in DataFrameReader
• Add unit tests for iceberg_tag time travel functionality
• Update AST proto with iceberg_tag field

Usage:

Direct parameter session.table("my_iceberg_table", time_travel_mode="at", iceberg_tag="v1")

Via DataFrameReader option (Spark-compatible) session.read.option("tag", "v1").table("my_iceberg_table")

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

   Fixes SNOW-NNNNNNN

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
   • If adding any arguments to public Snowpark APIs or creating new public Snowpark APIs, I acknowledge that I have ensured my changes include AST support. Follow the link for more information: AST Support Guidelines

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

   Please write a short description of how your code change solves the related issue.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 38.09524% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.95%. Comparing base (e56041f) to head (cdd1ba1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/snowflake/snowpark/dataframe_reader.py	25.00%	7 Missing and 2 partials ⚠️
src/snowflake/snowpark/session.py	0.00%	1 Missing and 1 partial ⚠️
src/snowflake/snowpark/table.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4211      +/-   ##
==========================================
- Coverage   95.42%   93.95%   -1.47%     
==========================================
  Files         171      171              
  Lines       43835    43857      +22     
  Branches     7513     7520       +7     
==========================================
- Hits        41829    41207     -622     
- Misses       1226     1859     +633     
- Partials      780      791      +11     

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[sfc-gh-aling]

Is the client change generated from the monorepo AST definition?

I remember a client AST change isn't strictly required right now; if it's needed, it should be derived from the monorepo updates -- there is a step by step doc for the AST mono repo updates.

@sfc-gh-heshah what's your recommendation here? can we just not add the ast in the PR and do that later?

[sfc-gh-aling]

I'm unable to see this feature in public doc.
what release status of this iceberg tag feature?

[sfc-gh-aling]

When using tag as an alias for iceberg_tag, is that mapping defined within the Snowflake Iceberg spec or it's a spark spec.

snowflake has its own "tag" concept, will there be future conflict under the context of dataframe reader?