[SPARK-34454][SQL] Mark legacy SQL configs as internal

[MaxGekk]

What changes were proposed in this pull request?

1. Make the following SQL configs as internal:
   • spark.sql.legacy.allowHashOnMapType
   • spark.sql.legacy.sessionInitWithConfigDefaults
2. Add a test to check that all SQL configs from the legacy namespace are marked as internal configs.

Why are the changes needed?

Assuming that legacy SQL configs shouldn't be set by users in common cases. The purpose of such configs is to allow switching to old behavior in corner cases. So, the configs should be marked as internals.

Does this PR introduce any user-facing change?

Should not.

How was this patch tested?

By running new test:

$ build/sbt "test:testOnly *SQLConfSuite"

[SparkQA]

Test build #135200 has finished for PR 31577 at commit 111635d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #135226 has started for PR 31577 at commit 84aed2f.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/39808/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/39808/

[srowen]

Seems OK to me if we're saying nobody should be able to set it? but would we want to forbid it?

[MaxGekk]

but would we want to forbid it?

If someone has a good reason to have a legacy conf as non internal, she/he can add an exception to the test. But I believe we should have a mechanism to catch the situation when someone adds "external" legacy config.

[dongjoon-hyun]

+1, LGTM. Thank you, @MaxGekk and all.
Yes, I also agree that this is good for consistency and will prevent any surprise in the future.
For the specific case, we can allow it specifically as @MaxGekk suggested.
Merged to master.