[WIP][SPARK-57421][SQL][CONNECT] Support @-syntax timestamp time travel on table names

[sotikoug83]

What changes were proposed in this pull request?

Add the @ time travel shorthand on table names so that

SELECT * FROM name@vN
SELECT * FROM name@<TS>

and

spark.read.format("format").table("name@vN")
spark.read.format("format").table("name@<ts>")

parse into the same plan as the existing VERSION AS OF / TIMESTAMP AS OF and option("versionAsOf") / option("timestampAsOf") clauses. Support for SQL, the DataFrame reader, and Spark Connect.

Why are the changes needed?

Spark has SQL AS OF and the versionAsOf/timestampAsOf reader options but not the compact @ suffix. Resolving it at parse time also simplifies the time travel entry points pipeline.

Does this PR introduce any user-facing change?

Yes, new syntax on table names gated by a conf (spark.sql.timeTravel.atSyntax.enabled) which allows time travel, default true. No longer gives a parse error when enabled and the syntax is used. Additionally, different from TIMESTAMP AS OF <TS> (which allows for arbitrary expressions in <TS>), the @<ts> timestamp only accepts the fixed 17-digit yyyyMMddHHmmssSSS Delta convention.

How was this patch tested?

New tests in PlanParserSuite, DataSourceV2SQLSuite, and SparkConnectPlannerSuite.

Was this patch authored or co-authored using generative AI tooling?

With help from Claude :)