Set User-Agent header on all outbound requests

[dbluhm]

This is a successor to #2781 which addressed missing (or default) User-Agent headers causing 403 errors with some domains. Notably, some services block requests from default User-Agents from certain origins. #2781 specifically addressed this for JSON-LD context loading. This issue captures the intent to add User-Agent headers to all of our outbound requests.

Outbound Requests:

• JSON-LD Document loader
• DIDComm Message delivery
• Webhooks
• Others?

It may be desirable to make the specific user agent used by ACA-Py configurable.