thrift/thriftutil/enums: avoid duplicating underscores

[xgoffin]

What does this PR do?

⚠️ NOTE: this is actually to patch the behaviour of the underlying CamelCaseToSnakeCase func. I could have put it in that func directly, however:

• It's called CamelCaseToSnakeCase, and Camel Case does not accept underscores. It kind of makes sense that it'd cause issues with underscores then
• If we start planning for underscores then, won't we have more bad surprises later on?
• It makes more sense to patch its output in the thrift func where the case is not exactly respected imo

Can easily change it back if needs be

Fixes #

What are the observable changes?

Good PR checklist

• Title makes sense
• Is against the correct branch
• Only addresses one issue
• Properly assigned
• Added/updated tests
• Added/updated documentation
• Properly labeled

Additional Notes

[linear]

DRA-2039 [upfluence-if][identity-server][activity-server] 2FA-related mixpanel events

[Sypheos]

That's indeed something that changed overtime. We usually have have lower case letters in the enums now.

We could have a bunch of helpers that are not camelCaseToSnakeCase but more like SnakeCaseEnum that do indicate we don't assume the input format