Feat/add suport for scoped oauth

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

[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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PagerDuty / backstage-plugin