[go] wrpcnats async parameter write indexing

[rvolosatovs]

Currently Index method is not implemented in wrpcnats.paramWriter, because as part of the root subject write it may lazily perform the handshake.
The implementation should be updated in such a way that the indexed parameter writers would not be able to write until the root writer performs the handshake. The transmission subject would then need to be communicated to the indexed writers.

Perhaps a possible way to handle this would be to use a RWMutex, which is locked on the root writer creation and unlocked once the handshake is complete. Indexed parameter writers Writes would then start with RLocking that shared mutex and block until the handshake is complete. We could also change init field to atomic.Bool here perhaps, which would let the indexed writers first check the atomic and only then attempt to RLock is they loaded false

https://github.com/wrpc/wrpc/blob/a75b04391524217058fb4c5284aee224ee8e4464/go/nats/client.go#L193

[rvolosatovs]

Rust implementation of this looks like this:
https://github.com/rvolosatovs/wrpc/blob/85b043b84bd70fa2994cde8ced129d05b6739284/crates/transport-nats-next/src/lib.rs#L444-L467
https://github.com/rvolosatovs/wrpc/blob/85b043b84bd70fa2994cde8ced129d05b6739284/crates/transport-nats-next/src/lib.rs#L514-L533

So on successful handshake, the negotiated subject is sent to all indexed parameter writers and the error writer.