feat: Add Spark SQL parser dialect config

[kumarUjjawal]

Which issue does this PR close?

• Closes Add support for Spark SQL dialect #21653.

Rationale for this change

This lets users combine Spark-compatible functions with Spark SQL parsing when they choose the dialect.

What changes are included in this PR?

• Add Spark to the SQL parser dialect config enum.
• Accept spark and sparksql as dialect config values.
• Use Spark dialect for Spark sqllogictests.
• Keep with_spark_features() config-neutral.
• Update dialect docs and expected config output.
• Add tests for Spark dialect parsing and Spark SQL execution with Spark functions.

Are these changes tested?

Yes

Are there any user-facing changes?

Users can now set datafusion.sql_parser.dialect to spark or sparksql.