BFD-4205: Refactor xref and kill credit handling

[aschey-forpeople]

JIRA Ticket:
BFD-4205

What Does This PR Do?

• Combines the bene_hstry and bene_xref tables
  • The kill_credit code is now checked as part of computing the current xref_sk
  • This allows us to handle kill_credit directly in the pipeline and database without the server having to know about it
  • Also fixes searching on historical MBI/bene_sk where the xref relationship should not be honored
• Fixes search by historical MBI
  • Per the spec, searching on a historical MBI should return the current bene_sk with the historical identifiers populated
• More organization of test data folders
• Move coverage query to separate CoverageRepository
• Fixes missing MBI start dates for current MBIs
• Refactors identity handling to use a database view
  • This allows us to correctly map MBI to an optional string, since MBIs are only optional for historical data
  • Converters aren't applied when model fields are used as parameters, so we can't make MBI optional on the main beneficiary model

What Should Reviewers Watch For?

If you're reviewing this PR, please check for these things in particular:

What Security Implications Does This PR Have?

Please indicate if this PR does any of the following:

• Adds any new software dependencies

• Modifies any security controls

• Adds new transmission or storage of data

• Any other changes that could possibly affect security?

• I have considered the above security implications as it relates to this PR. (If one or more of the above apply, it cannot be merged without the ISSO or team security engineer's (@sb-benohe) approval.)

• I have created tests to sufficiently ensure the reliability of my code, if applicable. If this is a modification to an existing piece of code, I have audited the associated tests to ensure everything works as expected.

Validation

Have you fully verified and tested these changes? Is the acceptance criteria met? Please provide reproducible testing instructions, code snippets, or screenshots as applicable.

tests

[brick-green]

no notes :)