atrium-streams + atrium-streams-client implementation

[oestradiol]

This Pull Request adds two new crates:

• atrium-streams: Definitions for traits, types and utilities for dealing with event stream subscriptions.
• atrium-streams-client: A library that provides default implementations of the EventStreamClient, Handlers and Subscription defined in atrium-streams for interacting with the variety of subscriptions in ATProto.

TODO:

• [x] - Implement processing of other payload types for Firehose implementation + add configurations to manually enable payload types to avoid unnecessary computations;
• [x] - Write unit tests;
• [ ] - Implement other subscriptions, such as com.atproto.label.subscribeLabels + tests;
• [ ] - Implement an alternative client for codebases that do not use Tokio as an asynchronous runtime + tests.
• [ ] - Create crate features so that users can deactivate specific features;
• [ ] - Write CHANGELOG.md and README.md;
• [ ] - Write Github Workflows for both crates + test features;
• [ ] - Prepare for wasm target;
• [ ] - Using std::mem::transmute is not recommended. It works wonderfully for now, but eventually it might not anymore. Think of a way to replace the outdated rs_car crate somehow (if possible).