An Elixir atproto OAuth and XRPC client

Fall back to PDS on Slingshot failure #40

Merged jola.dev
targeting main
Merged opened by and assigned to jola.dev
targeting main

To make the Slingshot integration less of a single point of failure, this makes us fall back to the PDS when Slingshot is down or suffers from intermittent issues.

Also consistently set a user agent.

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