sysvar: add system variable tidb_opt_enable_hash_join (#15105)

[ti-chi-bot]

This is an automated cherry-pick of #15105

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What is the related PR or file link(s)?

• This PR is translated from:sysvar: add system variable tidb_opt_enable_hash_join docs#14905
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• Delete files
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@qw4990: adding LGTM is restricted to approvers and reviewers in OWNERS files.

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[hfxsd]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #15586.