Add OnMemChange to support memory tracing of LockCtx

[RemHero]

What problem does this PR solve?

Issue Number: close #51217

Problem Summary:
The memory consumed in InitCheckExistence is not tracked by the memory tracker.Related PR.

What changed and how does it work?

This PR adds the OnMemChange method to InitCheckExistence. In addition, two functions GetValuesKSize and GetValuesVSize are added to account for the memory occupied by Key(string) and Value(ReturnedValue) in the declared map structure.

Check List

Tests

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

  • I checked and no code files have been changed.

[RemHero]

@ekexium
Hello! I have submitted client-go #1301 and TiDB pingcap/tidb#52739. It should be noted that the structure of LockCtx in client-go is changed, but the client-go version referenced in TiDB is v2. Even if PR in client-go is merged, it cannot make TiDB perceive the structural change of LockCtx.