Add conditional compilation support for `iml_runtime_apis!` (#14709)

* Handle `cfg_attr` in `decl_runtime_api`

* Integration tests

* UI tests

* Enable staging api tests on CI

* docs

* Comments and minor style fixes

* Fix doc comments

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* Fix formatting and a compilation error

* Fix a doc comment

* Combine the errors from `ApiImplItem`

* Fix a typo in UI test

* Use attribute span when reporting multiple conditional `api_version` error

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <git@kchr.de>

* Don't use `cfg_attr` for methods. Use simple feature gate instead

* Remove a stale comment

* Update primitives/api/proc-macro/src/impl_runtime_apis.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Revert "Update primitives/api/proc-macro/src/impl_runtime_apis.rs"

This reverts commit 4da20a79bd762580ebf502e9f807c2d7e5876106.

* Code review feedback

* Style improvements

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: parity-processbot <>