Allow config to be partially updated

[RealOrangeOne]

Description

Allow configuration to be partially updated.

Motivation and Context

This allows only some configuration variables to be changed, whilst leaving the defaults in-tact.

Types of changes

Put an x in all boxes that apply

• Bug fix (non-breaking change which fixes an issue) (kinda)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Put an x in all boxes that apply
If you're unsure about any of these, don't hesitate to ask. We're here to help!

• My code follows the code style of this project.
• I have added/updated docstrings to the code I have touched.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the Changelog to reflect my changes
• Has the changes been tested against a real Slack instance?

[RealOrangeOne]

Test failures don't look related to this PR

[sedders123]

Thanks for this! I agree test failures are not related. Will merge this and deal with CI in another PR.