feat: Tokens for use with the REST API

[jesuino]

Provide a clear and concise description of the feature request

As user I want to interact with the REST APIs from my third part application, but currently it is not possible to generate a token from UI or from other endpoint.

The backstage UI sends a bearer token for all API requests, but users currently can't do it without inspecting requests from Backstage UI:

Describe the problem that this feature resolves

Allow third part applications to consume backstage REST API.

Additional context or screenshots for the feature request

Acceptance Criteria

A documented endpoint or UI screen should allow users to generate tokens that can be used with the REST API.

[jesuino]

Related documentation:

https://github.com/backstage/backstage/blob/master/contrib/docs/tutorials/authenticate-api-requests.md
https://backstage.io/docs/auth/#configuring-token-issuers