Propagate contexts down scaler call stacks

[arschles]

This PR begins to propagate context.Contexts down the call stacks of various scalers.

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - learn more)

• Tests have been added

• Update all http.NewRequest() calls to http.NewRequestWithContext()

• Update calls to functions in the redis-go module to use contexts passed from callers, rather than just context.Background()

• Update calls to PostgreSQL/MySQL/MSSQL Scaler (due to SQL's QueryRowContext) to contexts passed from callers

• Same as ^^ but for MongoDB

• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified) (N/A)

• A PR is opened to update the documentation on (repo) (if applicable) (N/A)

• Changelog has been updated

Partially addresses #2190

[JorTurFer]

Nice job! :)

[zroubalik]

Looking good! Thanks for doing this :)

[arschles]

@zroubalik @JorTurFer I believe this PR is ready for reviews, so I've moved it out of draft mode.

the changes are large when measured by number of files changed, but relatively small when measured by lines changed. that's because it's a largely mechanical change that ensures that context.Contexts are passed down the stack from the Scaler interface to the implementations of each scaler, and not manually created wherever possible. I tried to ensure that there is no net-new implementation code in this PR.

can you take a look when you get a chance?

[zroubalik]

Looking good! Just minor nit in Changelog.