Upgrade apis to v2 & allow to select apis via feature flag for optimized compile times

[sprudel]

This is achieved by the following steps:

• generate clients including feature flag
• guard individual apis with features flags
• switch from v2beta to v2 apis

[sprudel]

@buehler getting this PR reviewed fast would be highly appreciated. Happy to discuss any changes/adaption needed :-)

[buehler]

Hi @sprudel

Thanks for your contribution!
I really like the changes, especially since everybody is able to selectively use APIs. What I don't understand is the protocc_insertion_deletion_point for features in the cargo.toml file. Can you elaborate what happens there?

Also: Testing/Linting seems to be angry with some changes. Otherwise LGTM.

[sprudel]

Hi,
thanks a lot for the quick feedback.

https://docs.rs/protoc-gen-prost-crate/0.4.1/protoc_gen_prost_crate/#cargo-manifest-template describes the mechanism.
Basically gen_crate allows to generate the feature flags which need to be added to the Cargo.toml. During generation it must be known where to put the features. This is the reason for these special comments.

I have considered moving the generation to a separate crate, but this would probably make the publishing process more complex.

[sprudel]

@buehler I fixed the clippy warnings, should be good now.
I plan to tackle the comments of the other mr this evening.