Tor/SOCKS5 proxy support for Dart Solana RpcClient

[sneurlax]

Is your feature request related to a problem? Please describe.
The Solana RpcClient should optionally support Tor/SOCKS5 proxies.

Describe the solution you'd like
An optional parameter accepting a proxy address (and port).

Describe alternatives you've considered
We could set up a local HttpClient (or similar) which would forward traffic to a Tor/SOCKS5 proxy and use that as our RpcClient node address, but not all platforms support local HttpClients equally (some need non-HTTPS options enabled, etc.)

Additional context
Stack Wallet wants to add Solana, but wants it to use Tor if that preference is enabled.