fix(sdk): require HTTPS for bearer tokens

[felix314159]

Summary

• reject plaintext RPC URLs when a bearer token is configured
• preserve plaintext endpoints for clients without transport credential
• document the HTTPS requirement on the async and blocking network prover builders

Motivation

NetworkProverBuilder supports both custom RPC URLs and bearer-token authentication. before this pr, combining a bearer token with an http:// RPC URL caused the SDK to send the reusable Authorization credential over a plaintext HTTP/2 connection, so a malicious network observer would have been able to recover the token and use any privileges it grants until it expires or is refreshed..

the endpoint already requires HTTPS for mTLS client identities, so this change applies the same transport requirement to bearer tokens and returns an error before creating the plaintext connection.