improv(ux): Adding support for escalation policy dropdown on custom action

[t1agob]

Description

This PR introduces the capability of replacing the Escalation Policy ID input in the 'pagerduty:service:create' action with a drop-down with the Escalation Policies available to the user. It does so by exposing a local API that gets all escalation policies and returns a list of names and ids.

It introduces a new configuration option (apiToken) documented as part of Backstage configuration schema. Without it the plugin is unable to start but I have implemented proper warnings and error message to warn the users. It is also documented in the official documentation.

Issue number: #2

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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