Add session utils for authentication and authorization using OpenID Connect (OIDC)

[hackermd]

This PR implements utilities to create authorized requests.Session objects using OpenID Connect (OIDC).

It introduces two public classes that are both derived from requests.Session:

• ConfidentialClientSession for confidential clients using the OAuth 2.0 Client Credentials grant type
• PublicClientSession for public clients using the OAuth 2.0 Authorization Code grant type with Proof Key for Code Exchange (PKCE) challenge

It further adds the utility function create_session_from_client_credentials() that facilitates the creation of instances of the above classes given corresponding client credentials (ConfidentialClientCredentials or PublicClientCredentials).