SessionMiddleware: support FIPS builds without SHA-1

[bcarroll]

itsdangerous includes a patch for this issue, but the TimestampSigner used in starlette.middleware.sessions.SessionMiddleware does not allow the digest_method to be changed from the itsdangerous default, which is SHA1, and causes SessionMiddleware to generate an exception in a ubi8/ubi9 RedHat container.

Suggested fix:
add digest_method to SessionMiddleware constructor. To allow the developer to change the digest_method used by the TimestampSigner object (SHA256, SHA384, etc...)

[bcarroll]

Pull Request for this fix: #2986