Breakout dependency sets into extras, establish a better pattern for adapters with missing imports

[timkpaine]

Moved from #35 to better facilitate back-and-forth.

We haven't yet decided whether we want to push for adapters to all be part of csp proper, or if we want/expect adapters to be maintained independently. Regardless, we currently have C++-based parquet and kafka adapters, python-based db, csv, perspective, websocket, symphony, and slack adapters, with plans for C++-based redis, andwebsocket adapters, so it seems likely that we will have a reliance on lots of python libraries.

We will want to establish a (better) pattern for optional dependency sets, at the very least moving optional dependencies to their own extras blocks in pyproject.toml, and having a more graceful handling at runtime when dependencies are missing.

[trallard]

This was discussed with @robambalu
and @ptomecek, we agreed:

• Will aim for a plugin-like approach for adapters vs having them all be part of csp
  • This will help with long-term maintenance (reducing the burden on csp maintainers, especially regarding dependencies compatibility)
  • This will also help with user experience, as otherwise, users would end up with a lot of dependencies and adapters that they do not want/need
• If there are (external) adapters that prove to be integral to csp and their users, then these could be added to csp (will need to define clear guidelines and document)
• To start with:
  • parquet and kafka should remain part of csp
  • the rest could be moved out; we can start with moving the slack and symphony adapters out of csp

[timkpaine]

Seems like clear guidelines should happen before moving things in/out?

From a dependency standpoint for pure python deps, db is far worse than the others, perspective is massively bit-rotted, websocket is very incomplete.

slack and symphony dependencies are optional today, and their tests are more complete than the others, so they actually have the least maintenance and dependency overhead.

[timkpaine]

WIP starting on a separate pure-python adapter