expression: fix casting REAL type to STRING type (#16975)

[ti-chi-bot]

This is an automated cherry-pick of #16975

What is changed and how it works?

Issue Number: Close #16974

ref pingcap/tidb#53129

What's Changed:

expression: fix casting REAL type to STRING type
- unify the behavior of converting real types to strings in tidb#53129 and tikv.
- use ryu lib in tikv to implement casting real type to str type.
- `if abs(num) >= 1e15 or (num is not zero and abs(num) < 1e-15)` then use decimal exponent format 

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

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

Release note

Fix the incorrect result when casting REAL type to STRING type

[ti-chi-bot]

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[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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