Refactor: use cal-itp/littlepay to check API access

[angela-tran]

Closes #1814

This PR refactors the check-api workflow to use cal-itp/littlepay instead.

Like the previous implementation, this approach checks if it can get an access token for the QA and prod environments. Additionally it does those checks for each participant, as defined in the job matrix.

Before merging

• Copy contents of config.yaml for this API from LastPass, and add as a repository secret named API_CHECK_CONFIG

Cleanup to do

• Delete the old secrets used in the previous implementation
• Re-enable the check-api workflow

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[thekaveman]

Wow, this is really clever to check out the Littlepay library inside the workflow and install from there!

Seeing that, I wonder if it might make more sense to have this check run in that repo instead?

I really like the new matrix configuration too 👍

[angela-tran]

Seeing that, I wonder if it might make more sense to have this check run in that repo instead?

I could see arguments for either repo. The workflow does send a Slack notification to the Benefits channel if there's a failure, so that might be a reason to keep it here. On the other hand, cal-itp/littlepay pretty much exists to be used in Benefits, so you could say that it's just as valid for it to send notifications to the Benefits channel.

I'll go ahead and merge it in to this repo. Let me know if you feel strongly about moving it.