feat: migrate apis to backend

[t1agob]

Description

This PR adds REST API endpoints for all operations executed from the frontend plugin that currently interact with the PagerDuty REST API directly. With this change we:

• Remove the dependency on the Backstage proxy
• Improve security by limiting the actions performed on the backend API
• Slightly increase the performance by limiting the data used by the frontend to the essential

Endpoints added

• /oncall-users - returns PagerDutyOnCallUsersResponse with list of users oncall
• /services - uses integration_key and returns PagerDutyServiceResponse with PagerDuty service information
• /services/:serviceId - returns PagerDutyServiceResponse with PagerDuty service information
• /services/:serviceId/change-events - returns PagerDutyChangeEventsResponse with list of last 5 change events for the defined service
• /services/:serviceId/incidents - returns PagerDutyIncidentsResponse with list of incidents for the defined service

With this change, the proxy configuration on app-config.yaml is no longer required.

Issue number: #13.

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
• [x] 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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