statistics: fix "data too long" error when dumping stats from table with new collation data (#27033)

[time-and-fate]

cherry-pick #27033 to release-5.1-20211012

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What problem does this PR solve?

Issue Number: close #27024

When storing a histogram for a new collation column, we store its collate key instead of the original value.
However (1) when loading the histogram into stats cache, we try to convert the bounds of the histogram to the FieldType of this column (2) the collate key of a value is usually longer than the original value.
So when doing the type conversion, the value may exceed the FieldType.Flen, which caused the "Data Too Long" error.

What is changed and how it works?

When loading stats from storage into stats cache, for String values, no longer convert them to the FieldType of this column.
Now we always convert them to TypeBlob to bypass length checks.

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:

• qw4990
• winoros

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[ti-chi-bot]

@time-and-fate: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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