Remove `sp_runtime::RuntimeString` and replace with `Cow<'static, str>` or...

Remove `sp_runtime::RuntimeString` and replace with `Cow<'static, str>` or `String` depending on use case (#5693)

# Description

As described in https://github.com/paritytech/polkadot-sdk/issues/4001
`RuntimeVersion` was not encoded consistently using serde. Turned out it
was a remnant of old times and no longer actually needed. As such I
removed it completely in this PR and replaced with `Cow<'static, str>`
for spec/impl names and `String` for error cases.

Fixes https://github.com/paritytech/polkadot-sdk/issues/4001.

## Integration

For downstream projects the upgrade will primarily consist of following
two changes:
```diff
#[sp_version::runtime_version]
pub const VERSION: RuntimeVersion = RuntimeVersion {
-	spec_name: create_runtime_str!("statemine"),
-	impl_name: create_runtime_str!("statemine"),
+	spec_name: alloc::borrow::Cow::Borrowed("statemine"),
+	impl_name: alloc::borrow::Cow::Borrowed("statemine"),
```
```diff
		fn dispatch_benchmark(
			config: frame_benchmarking::BenchmarkConfig
-		) -> Result<Vec<frame_benchmarking::BenchmarkBatch>, sp_runtime::RuntimeString> {
+		) -> Result<Vec<frame_benchmarking::BenchmarkBatch>, alloc::string::String> {
```

SCALE encoding/decoding remains the same as before, but serde encoding
in runtime has changed from bytes to string (it was like this in `std`
environment already), which most projects shouldn't have issues with. I
consider the impact of serde encoding here low due to the type only
being used in runtime version struct and mostly limited to runtime
internals, where serde encoding/decoding of this data structure is quite
unlikely (though we did hit exactly this edge-case ourselves
😅

).

## Review Notes

Most of the changes are trivial and mechanical, the only non-trivial
change is in
`substrate/primitives/version/proc-macro/src/decl_runtime_version.rs`
where macro call expectation in `sp_version::runtime_version`
implementation was replaced with function call expectation.

# Checklist

* [x] My PR includes a detailed description as outlined in the
"Description" and its two subsections above.
* [ ] My PR follows the [labeling requirements](

https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md#Process
) of this project (at minimum one label for `T` required)
* External contributors: ask maintainers to put the right label on your
PR.
* [ ] I have made corresponding changes to the documentation (if
applicable)

---------

Co-authored-by: GitHub Action <[email protected]>
Co-authored-by: Guillaume Thiolliere <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>