Add enums for Login Sources and Pub/Sub message topics

Came from discussion notes in https://github.com/bcgov/lear/pull/3865

There are some places where "BCSC" (or 'bcsc") is used that are hardcoded. Perhaps other login source values as well.

The queue services in many spots use hardcoded strings for message types as well. And have duplicated prefixes that could be extracted to enum as well.

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**Dev Notes**

Can search more, but comments in that PR point out some spots and valid values for login source.

For the pub/sub message topics see DBC queue service implementation for extracting prefixes to constants. Can extend that pattern to enums and replace in the DBC

Location of enum could be in User model for the login source, might need new one for pub/sub message types.

**Ensure no breaking changes and case-sensitivity issues introduced**

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**QA Notes**
In the places in the code these strings have been changed to enums, regression test existing behavior
- TBA locations of changes