fix: adding support for API base url override

[t1agob]

Description

This PR resolves an issue reported in backstage-plugin (#74) which prevents users from overriding the REST API base url (e.g. for EU based accounts). This feature was possible through the proxy configuration.

With this PR, users will be able to add a new configuration to the PagerDuty plugin in app-config.yaml like the example below.

pagerDuty:
  apiBaseUrl: https://api.eu.pagerduty.com     #defaults to https://api.pagerduty.com

Issue number: NA

Type of change

• 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

• I have performed a self-review of this change
• Changes have been tested
• Changes are documented
• Changes generate no new warnings
• 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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