Allow set `extraHeaders` when calling `completeAuthorizationCodeGrant`

[weareoutman]

Currently matix-authentication-service recognize device type by parsing the User-Agent request header when calling token exchange endpoint, while it can't recognize a Tauri App on MacOS (Tauri doesn't override user-agent by default like Electron, so it would results as a Unknown device.

OidcClient:processSigninResponse from oidc-client-ts already supports setting extraHeaders, it would be nice completeAuthorizationCodeGrant also accepts extraHeaders as a param.

Then I can customize my own header so that MAS will recognize it correctly:

completeAuthorizationCodeGrant(
  code,
  state,
  {
    'User-Agent': 'MyApp/1.0.0'
  }
)

[t3chguy]

Given User-Agent is dropped by some UAs it is unlikely something we'd want the headache of documenting: https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_request_header - once https://bugs.chromium.org/p/chromium/issues/detail?id=571722 is fixed then maybe