PMI: allow user to override PMI key value length

[yfguo]

Pull Request Description

This PR fixes the #6924. The SDSC Expanse seems to have a long key value length that is not compatible with the MPICH default.

This PR does not depends on #7008. Only rebased on it because the formatting error is failing pre-commit hook.

Author Checklist

• [ ] Provide Description Particularly focus on why, not what. Reference background, issues, test failures, xfail entries, etc.
• [ ] Commits Follow Good Practice Commits are self-contained and do not do two things at once. Commit message is of the form: module: short description Commit message explains what's in the commit.
• [ ] Passes All Tests Whitespace checker. Warnings test. Additional tests via comments.
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[yfguo]

test:mpich/ch4/ofi