Make `transport` optional (potential WASM support)

[dakom]

I believe this should be two steps:

1. Feature-gating a bit more granularly

default could remain as-is. Would be something like a change from this:

[features]
default = ["std", "client"]
client = ["std", "dep:tonic", "tonic/codegen", "tonic/transport", "tonic/prost"]
server = ["std", "dep:tonic", "tonic/codegen", "tonic/transport", "tonic/prost"]

to this:

[features]
default = ["std", "client", "transport"]
client = ["std", "dep:tonic", "tonic/codegen", "tonic/prost"]
server = ["std", "dep:tonic", "tonic/codegen", "tonic/prost"]
transport = ["tonic/transport"]

2. providing a way to tonic_build without the transport feature. I'm not sure if that's part of the proto-compiler tool

This comment looks relevant: https://github.com/hyperium/tonic/issues/1538#issuecomment-2049778792

I'd be happy to make a PR, since I was able to build a client partially with the feature-gating locally, but couldn't proceed with the proto-compiler tool (instructions don't work, or I did something wrong, it requires having a --ics and --nft param)

[dakom]

couldn't proceed with the proto-compiler tool

ah, missed the warning to run via scripts/sync-protobuf.sh, that works :)