Allow attaching OIDC access token to DRS requests

[marius-mather]

Allow attaching OIDC access tokens when fetching data via DRS. The OIDC access token to use can be specified in the file sources config, e.g.

- type: drs
  id: my-drs-server
  url_regex: ^drs://localhost:4500/
  http_headers:
    Authorization: Bearer ${user.oidc_access_tokens['oidc']}

this probably makes most sense in cases where there is only one OIDC provider (and OIDC login is enforced), but has been left open in case it's useful for other use-cases.

We also want to sure that the OIDC access token is still fresh when using it to fetch - so this PR adds the ability to refresh from the data fetch job.

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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