Set User-Agent header from ApiSettings AgentTag

[cgooding-va]

For cookieless embedding, the User-Agent Header on the request to acquire the session must match the embedded user's browser agent, which is not possible currently as there is no way to define that header.

This change allows a User-Agent header to be set when making requests with the AuthSession service.

Fixes #1274

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[jeremytchang]

Thanks for the PR. I've copied your commit/changes over to another PR so our CI tests will run correctly. #1285 Closing this PR.