*: support prepared plan cache for cached table

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #32003

Problem Summary:

Make the cached table been able to use prepared plan cache.
Without plan cache, the performance decrease too much and counteract the benefit from table caching.

What is changed and how it works?

In the past, whether to read from table cache is decided in the logical plan optimize phase.
If the cache data is available, a UnionScan is added as the parent plan.

The problem of that implementation is, with / wo table cache, the generated plan is different.
That means the plan can't be reused (the shape of the plan may change)

In this PR, to leverage prepared plan cache, I defer the UnionScan generation to the executor phase.
In executor.build(), if table cache is available, TableReader will be changed to UnionScan->TableReader.

Most of the test code changes are unrelated...
Before this PR, we use "explain select ..." and check the UnionScan to verify that the table cache data is used.
Now we can't do that because there will be no UnionScan in explain result any more.
So the test code is updated.

Another unrelated change is that the "Cannot use IndexMerge on TableCache." warning is gone.
I don't mean to do that but since the code logic change, the restriction is temporarily removed.
There is already an issue for it #29682 and I'll fix later.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lcwangchao
• tangenta

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/5ece89b572983d6cb8c0fde084a3aa2c65e914be

[tangenta]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 12ece11

[ti-chi-bot]

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[tiancaiamao]

/merge

[tiancaiamao]

/run-unit-test