feat: basic jetstream consumer app

[malpercio-dev]

Rust has been chosen to build out this AppView, as we expect that even a moderately popular forum could see quite a bit of traffic, and therefore must be performant.

While far from what atBB will be, this PR is intended to lay some basic groundwork necessary to begin building out the server application, including:

• a simple Jetstream subscriber, which will be preferred over ingesting the firehose or hosting a relay, in support of ease of deployment of atBB
• a CLI that can be used to create a Jetstream subscription and observe the output
• setup of Docker, again in support of ease of deployment of atBB, and a simple GitHub Actions CI workflow to publish the image to ghcr.io