Add conditional compilation support for impl_runtime_apis!

[tdimitrov]

This PR handles cfg_attr attribute for iml_runtime_apis!. It allows conditionally to compile different runtime api version implementations based on feature flag. E.g.:

pub struct Runtime {}
decl_runtime_apis! {
pub trait ApiWithStagingMethod {
    fn stable_one(data: u64);
    #[api_version(99)]
    fn staging_one();
}

impl_runtime_apis! {
#[cfg_attr(feature = "enable-staging-api", api_version(99))]
impl self::ApiWithStagingMethod<Block> for Runtime {
    fn stable_one(_: u64) {}

    #[cfg(feature = "enable-staging-api")]
    fn staging_one() {}
}

The code above will compile version 1 (with just stable_one) by default and version 99 (with stable_one and staging_one) if enable-staging-api feature is enabled.

EDIT: modified the sample code to match the latest implementation suggested by @bkchr

[tdimitrov]

Is this a good idea?

[rphmeier]

I would suggest not to do this for now, as runtime API versions often activate experimental features which may be buggy. It's acceptable for experimental features to be buggy in master, but it should not lead to test failures.

In the PR that stabilizes experimental APIs, we'd encounter the test failure in CI anyway.

[tdimitrov]

I haven't marked the correct line. It doesn't run all tests with enable-staging-api but only sp-api-test:

time cargo nextest run -p sp-api-test --features enable-staging-api

I've got some runtime api unit tests with feature flags, but they need to be invoked individually.

[bkchr]

Ty! Looks good, just some last nitpicks :)

[bkchr]

bot rebase

[paritytech-processbot]

Rebased

[tdimitrov]

bot merge