Unify Snowflake object name handling in the Snowpark AST

[sfc-gh-oplaton]

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

   Fixes SNOW-1621205

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.

This is the client-side change corresponding to https://github.com/snowflakedb/snowflake/pull/240557

Unify Snowflake object name handling in the Snowpark AST.
Remove FnName and SpTableName. They both had Flat and Structured variants, but ultimately designate Snowflake object names.
Introduce data SpName and entity SpNameRef for referring to Snowflake objects by relative or fully qualified name.
Update FnNameRefExpr.
Use SpNameRef in a few places that used to use List[String].

[sfc-gh-vbudati]

This looks like it's nested twice -- is this intentional?