Allow set `extraHeaders` when calling `completeAuthorizationCodeGrant`

Nobody has claimed this yet.

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:

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

Start at completeAuthorizationCodeGrant and compare its parameter handling with oidc-client-ts's OidcClient:processSigninResponse, which already supports extraHeaders. Trace the authorization-code token exchange entry point and verify that supplied headers can reach it, including the custom User-Agent example described in the issue.

Written by the indexing model from the issue text.