[SPARK-52428] Add DataFrame::merge_into() builder for MERGE INTO operations

[rafafrdz]

Summary

• Add MergeIntoBuilder with chainable methods: when_matched_update(), when_matched_delete(), when_not_matched_insert()
• Add DataFrame::merge_into(target_table, condition) returning the builder
• execute() creates a temp view for the source DataFrame and generates MERGE INTO SQL

df.merge_into("target_table", "target.id = source.id")
    .when_matched_update("target.value = source.value")
    .when_not_matched_insert("*")
    .execute()
    .await?;

Test plan

• cargo build passes
• cargo clippy passes
• cargo fmt -- --check passes
• Integration test with Delta Lake table