[SYNPY-1570, SYNPY-1555] Support passing user_agent to the Synapse class constructor, and HTTPX Library usage

[BryanFauble]

Problem:

1. When making HTTP requests with the HTTPX library, we were incorrectly including the User-Agent header from the Python 'requests' package instead of using HTTPX's own User-Agent.
2. It was cumbersome for users to specify their own additional User-Agent fields to add to the HTTP requests.

Solution:

1. Replacing the requests package User-Agent header for requests that are using the HTTPX library.
2. Provide a class argument for the Synapse class that allows users to specify their own User-Agent headers to add.

Testing:

1. Unit testing the logic for both creating valid/invalid headers, along with verifying that the headers are correct getting added on the outbound requests for the requests and httpx library.