config: Fix titan blob-run-mode setting (#15988)

[ti-chi-bot]

This is an automated cherry-pick of #15988

What is changed and how it works?

Issue Number: Close #15987, Close #15978

What's Changed:

Fix titan config blob-run-mode's from<ConfigValue> implementation.  

Related changes

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

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• tonyxuqqi
• v01dstar

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[v01dstar]

LGTM

[tonyxuqqi]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: 610c2cc