feat: entity mapping persistence

Description

This PR adds support for new entity mapping APIs to be used by the frontend component to allow easy onboarding of existing services from PagerDuty into Backstage. This includes new API endpoints to:

Get all entity mappings persisted to the database
Get an entity mapping for a specific Backstage Entity reference
Persist mappings into the database

This PR also introduces the needed mechanisms to persist data into the database selected by the Backstage Admin. The database table is automatically generated and is exclusively used by PagerDuty plugin Backstage to ensure data segregation.

Issue number: N/A

Type of change

[x] New feature (non-breaking change which adds functionality)
[ ] Fix (non-breaking change which fixes an issue)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

[x] I have performed a self-review of this change
[x] Changes have been tested
[ ] Changes are documented
[x] Changes generate no new warnings
[x] PR title follows conventional commit semantics

If this is a breaking change 👇

[ ] I have documented the migration process
[ ] I have implemented necessary warnings (if it can live side by side)

Acknowledgement

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PagerDuty / backstage-plugin-backend