feature: amend garak user-agent

[leondz]

We should be good citizens and not use the default requests user-agent, but instead our own

• add new run-level config param for useragent
• use this in generators using requests library

Verification

List the steps needed to make sure this thing works

• drawing a blank here for now. maybe when this comes out of draft, we can use requests-mock

[erickgalinkin]

We may want to find other places where we're using requests and fix the user agent -- may want to place the user agent in the shared resources so we can ref it easily.

[jmartin-tech]

Consider finding a hook to set for all uses in common libs we are already dependent on:

• requests
• aiohttp
• httpx

[erickgalinkin]

LGTM

[jmartin-tech]

Minor tweaks.