[redis-plus-plus] Support async interface with features

[elmanelman]

Is your feature request related to a problem? Please describe.
For now it isn't possible to choose whether to install and build async part of redis-plus-plus with vcpkg. These features are enabled with cmake variables: REDIS_PLUS_PLUS_BUILD_ASYNC=libuv and, optionally, REDIS_PLUS_PLUS_ASYNC_FUTURE=boost, if you want to use boost::future instead of std::future.

Proposed solution
Add library features that, being selected, will set ...BUILD_ASYNC and ...ASYNC_FUTURE variables.

For instance, redis-plus-plus[async] (or [async-std]) could set ...BUILD_ASYNC=libuv only, and redis-plus-plus[async-boost] could set both ...BUILD_ASYNC=libuv and ...ASYNC_FUTURE=boost.

Describe alternatives you've considered
I couldn't find any other concise ways to solve this.

[JackBoosY]

According to our policy, we refuse to accept adding conflicting features. So do you need std or boost?

[elmanelman]

It would be nice to set ...BUILD_ASYNC and ...ASYNC_FUTURE separately, as library users may want to choose which futures to use and, looking ahead, choose event loop lib.

I suggest adding two features (meanwhile, I will be glad of better solutions):

• async-libuv (or async-event-loop-libuv) that sets REDIS_PLUS_PLUS_BUILD_ASYNC=libuv
• async-futures-std that sets REDIS_PLUS_PLUS_ASYNC_FUTURE=std
• async-futures-boost that sets REDIS_PLUS_PLUS_ASYNC_FUTURE=boost

So, you could pick redis-plus-plus[async-libuv,async-futures-std] for libuv event loop & std futures, and redis-plus-plus[async-libuv,async-futures-boost] for libuv & boost futures.

[JackBoosY]

Which boost components is required by async-futures-boost?

[JackBoosY]

@elmanelman According to our doc, we can only pick one feature, std or boost.

[elmanelman]

Hmm, I got it and think we can leave async & async-std for now, as you implemented in #25272.

But what is the right way to give a choice between std and boost futures?

[JackBoosY]

If you need to use async-futures-boost, you may need to modify vcpkg.json(add feature and add dependency items boost components) and portfile.cmake(check the feature whether it's enabled and set -DREDIS_PLUS_PLUS_ASYNC_FUTURE=boost to vcpkg_configure_make to enable it) using overlay port feature.
See https://github.com/microsoft/vcpkg/blob/master/docs/specifications/ports-overlay.md.

[elmanelman]

Awesome, thanks!