SDK crate split & new crate structue

[marvin-hansen]

As discussed on Discord and outlined in issue #1713, its time to axe the Rust SDK mega-crate, slice and dice it into a number of smaller crates.

Please discuss thoughts, ideas, and everything related to the SDK split here.

[spetz]

The most important parts that come to my mind currently are:

• Move the shared data models (commands, responses, underlying data structures, client errors) to the new, lightweight shared SDK which could be referenced by both, server and existing Iggy SDK
• Cleanup the server errors, as some of them are part of the public SDK error enum type, while these are only used internally by the server

[marvin-hansen]

For now, the proposed crates are:

• iggy_error (i.e. iggy_server_error, iggy_client_error, etc)
• iggy_types (type and trait definition used in server and SDK)
• iggy_common (most likely contains functionality, serialization, and utils shared in server and SDK)
• iggy_protocol (The binary protocol and its default implementation)
• iggy_sdk (the trimmed down SDK crate. Might keep the original iggy name if that prevents to much breaking)

I tend to separate types from errors mainly to re-used errors in all sorts of crates without a long tail of unrelated deps. I think that also helps to clean up the server errors by simply creating maybe 2-3 error crates to fully isolate errors by domain, add unit testing and call it a day.

Any other ideas or thoughts?