Add support for tokens in upstream headers

[sevensolutions]

This PR adds support for sending the access tokens to the upstream service by using custom headers as described in #36.

Please note that this will be a breaking change in the Header-configuration.
Previously, sending claim-values to the upstream was done like this:

Headers:
  MapClaims:
    - Claim: "preferred_username"
      Header: "X-Oidc-Username"
    - Claim: "sub"
      Header: "X-Oidc-Subject"

Which now needs to be written as:

Headers:
  - Name: "X-Oidc-Username"
    Value: "{{`{{ .claims.preferred_username }}`}}"
  - Name: "X-Oidc-Subject"
    Value: "{{`{{ .claims.sub }}`}}"

But in addition, this now also support sending tokens:

Headers:
  - Name: "Authorization"
    Value: "{{`Bearer {{ .accessToken }}`}}"