split connector implementations into crates

[xxchan]

BTW, I think it's time to split connector implementations off and design a clear interface for it, as there are now community developers willing to develop new connectors. 🤔

Originally posted by @wangrunji0408 in #16759 (comment)

related work #12981

related discussion:

• But default features is to make developers happy, which can be hard to fix.

The UDF module is touched by few people, so it makes sense to require manual enabling on the features.

However, imagine we're going to add conditional compiles for the connector module, which is actively developed and maintained by a considerable amount of developers. We still need the effort to inform them how to enable these features, especially the IDE part, which is not better than the approach above.

The point is the most of developers do not pay much attention to the compile time. If something comes naturally, it's good. If it requires extra effort, they might give up. Thus in my opinion, it's fine to treat it as a hidden treasure needed to be discovered by some manual efforts. 😄

Originally posted by @BugenZhao in #16759 (comment)

The point is the most of developers do not pay much attention to the compile time. If something comes naturally, it's good. If it requires extra effort, they might give up.

I 100% agree. However, my opinion from this fact is that it's better to offer "free" fast compile time for developers. If they are not good at it, we'd better make the default faster. Saving every one's time does good to the whole world!

Originally posted by @xxchan in #16759 (comment)

how to enable these features, especially the IDE part

I'm thinking whether it's possible to offer IDE experience without config for feature flags. e.g., cfg_or_panic(madsim).

One idea is that: we put each connector impl in separated crate, which is an optional dependency. If so, maybe the IDE can work without config.

Originally posted by @xxchan in #16759 (comment)

[xxchan]

From the diff in this PR, it seems the boundary is quite clear and it should be easy to split them out. #17154

The question is just which plan do we choose: feature flags? separated crates?