sessionctx: move set variable to sysvar struct

[morgo]

What problem does this PR solve?

Problem Summary:

This is a refactor only.

Previously, setting a sysvar used a large switch statement to run the required code to process the change. In this PR, the switch statement is refactored to be a function call on the sysvar struct. The advantage of doing this as a function call, is that plugins will be able to add/process setvar changes in the same way as regular server options.

Additional notes:

• Validation already calls a function on the sysvar struct. This change moves the SetSession function to the struct.
• There are some special-cases for character set, which for now continue to be handled as part of the switch statement.

What is changed and how it works?

What's Changed:

Code refactored

Related changes

• None

Check List

Tests

• Unit test

Existing tests pass

Side effects

• Large code changes (makes merging/cherry picking hard.)

Release note

• No release note

[morgo]

/run-all-tests

[morgo]

/run-common-test

[morgo]

/run-all-tests

[morgo]

/run-common-test

[morgo]

/run-check-dev-2

[morgo]

/run-all-tests

[morgo]

/run-check_dev_2

[morgo]

There was a test failure that is fixed in #21442 - I've added the code from this PR here, but it will dissapear from the diff assuming it merges to master soon.

[morgo]

/run-all-tests

[morgo]

Closing for now.