cosmos-sdk-proto: Generated `serde` serialization code is not usable in `no_std` context

[romac]

It looks like the code generated by the buf.build/community/neoeinstein-prost-serde:v0.3.0 is not usable in no_std context:

$ git clone git@github.com:cosmos/cosmos-rust
$ cd cosmos-rust/cosmos-sdk-proto
$ git checkout cosmos-sdk-proto/v0.25.0-pre.0
$ cargo check --no-default-features --features serde
(...)

error[E0433]: failed to resolve: use of undeclared crate or module `std`
 --> cosmos-sdk-proto/src/prost/cosmos-sdk/cosmos.auth.v1beta1.serde.rs:5:46
  |
5 |     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
  |                                              ^^^ use of undeclared crate or module `std`

(...)

error[E0433]: failed to resolve: use of undeclared type `ToString`
    --> cosmos-sdk-proto/src/prost/cosmos-sdk/cosmos.vesting.v1beta1.serde.rs:1191:50
     |
1191 |             struct_ser.serialize_field("length", ToString::to_string(&self.length).as_str())?;
     |                                                  ^^^^^^^^ use of undeclared type `ToString`
     |
help: consider importing one of these items
    -->  cosmos-sdk-proto/src/lib.rs:221:13
     |
221  +             use alloc::string::ToString;
    -->  |cosmos-sdk-proto/src/lib.rs:221:13
     |
221  +             use serde::__private::ToString;

(...)

To fix this, we should attempt to make the generated code work with no_std, either by patching the output, or by using informal-pbjson to generate the serialization code, like we do in ibc-proto-rs.

Just enabling the std feature when serde is enabled is unfortunately not really an option, as ibc-proto has downstream users which depend on the serde feature but not std.

As such it is currently blocking cosmos/ibc-proto-rs#187

[tony-iqlusion]

Should be fixed by #500.

I also opened an upstream issue: neoeinstein/protoc-gen-prost#102