Feat/add suport for scoped oauth

[t1agob]

Description

This PR adds the type necessary for OAuth support in Backstage plugin configuration. Users can now configure the following OAuth parameters in Backstage app-config.yaml file.

pagerDuty:
  oauth:
    clientId: ${PD_CLIENT_ID}
    clientSecret: ${PD_CLIENT_SECRET}
    subDomain: ${PD_ACCOUNT_SUBDOMAIN}
    region: ${PD_ACCOUNT_REGION}           // Optional. allowed values: 'us', 'eu'. Defaults to 'us'.

It also introduces more friendly error messages when permissions for certain APIs or operations are not present.

Issue number: #53

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

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Disclaimer: We value your time and bandwidth. As such, any pull requests created on non-triaged issues might not be successful.