variable: Add IsNoop to sysvar struct

[morgo]

What problem does this PR solve?

Problem Summary:

It should be possible to tell if a sysvar is a noop from the SysVar struct.

What is changed and how it works?

What's Changed:

A new field is added. This is used in this docs PR here to autogenerate sysvar docs:
pingcap/docs#5720

Related changes

• PR to update pingcap/docs/pingcap/docs-cn: Auto-generate sysvar documentation from TiDB server source docs#5720

Check List

Tests

• Unit test

Side effects

• None

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• wjhuang2016
• xiongjiwei

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[xiongjiwei]

/merge

[ti-chi-bot]

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

Commit hash: a1d64d9

[ti-chi-bot]

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