feat: Support optional headers for OAuth request (#1815)

* Supporting optional headers for oauth.

* Optional OAuth only Headers with tests for None

---------

Co-authored-by: Edgar R. M <[email protected]>

singer_sdk/authenticators.py

@@ -358,6 +358,7 @@ def __init__(
         auth_endpoint: str | None = None,
         oauth_scopes: str | None = None,
         default_expiration: int | None = None,
+        oauth_headers: dict | None = None,
     ) -> None:
         """Create a new authenticator.
 
@@ -366,11 +367,13 @@ def __init__(
             auth_endpoint: The OAuth 2.0 authorization endpoint.
             oauth_scopes: A comma-separated list of OAuth scopes.
             default_expiration: Default token expiry in seconds.
+            oauth_headers: An optional dict of headers required to get a token.
         """
         super().__init__(stream=stream)
         self._auth_endpoint = auth_endpoint
         self._default_expiration = default_expiration
         self._oauth_scopes = oauth_scopes
+        self._oauth_headers = oauth_headers or {}
 
         # Initialize internal tracking attributes
         self.access_token: str | None = None
@@ -499,6 +502,7 @@ def update_access_token(self) -> None:
         auth_request_payload = self.oauth_request_payload
         token_response = requests.post(
             self.auth_endpoint,
+            headers=self._oauth_headers,
             data=auth_request_payload,
             timeout=60,
         )
@@ -512,7 +516,8 @@ def update_access_token(self) -> None:
 
         token_json = token_response.json()
         self.access_token = token_json["access_token"]
-        self.expires_in = token_json.get("expires_in", self._default_expiration)
+        expiration = token_json.get("expires_in", self._default_expiration)
+        self.expires_in = int(expiration) if expiration else None
         if self.expires_in is None:
             self.logger.debug(
                 "No expires_in receied in OAuth response and no "