DEVPROD-13567: improve retry and traceability for GitHub app token creation errors

[Kimchelly]

DEVPROD-13567

Description

This is some improvements in preparation to fix the issue where github.generate_token will time out requesting a token from Evergreen. This will help to verify that the actual fix for DEVPROD-13567 works and overall improve our ability to monitor for issues when creating tokens.

I'm going to post a follow-up PR after this to address the ticket's issue.

• Rework GitHub app client to support a retry function. (This functionally should behave the same, just written in a more configurable/observable way.)
• Add OTel spans for better tracing of success/error when creating a token.
• Add Splunk logging for non-retryable errors from the GitHub app client.

Testing

Tested in staging that the spans went to Honeycomb and Splunk errors worked as expected with the retry function. Also verified that modifying the HTTP client's transport didn't cause token creation to fail.