Refactor & improve ClientBuilder

[partylikeits1983]

After opening #741, there are a few optimizations that can be made. The goal of #741 is to provide the option to make initialization of the miden-client as simple as possible. Here are the three main issues (as discussed in the comments of #741) that need to be addressed to optimize the current implementation of the ClientBuilder.

1. Make handling of KeyStore more generic (so that we could use non-FilesystemKeyStores as well).
2. Make handling of Rng more generic (so that we could use non-RpoRandomCoin as well).
3. Modifying rpc/tonic_client/mod.rs to avoid having the #[cfg(feature="std")] guard for the ClientBuilder mod