Provide a sync + non-blocking API when server-side evaluation is enabled

We'd like to use this SDK in a variety of different services were performance matters. We almost only use it with local side evaluation and in different contexts, sync, async and sometimes in async contexts but with manual future implementation, for example when implementing tower services (https://docs.rs/tower-service/latest/tower_service/).

It appears that if server side evaluation is enabled there is no need to do any fetch eagerly. Cf this related code:
```rust
if flagsmith.options.enable_local_evaluation {
// Update environment once...
if let Err(e) = update_environment(&client, &ds, &environment_url) {
log::warn!("Failed to fetch environment on initialization: {}. Will retry in background.", e);
}
```
with `update_environment` writing to the the `eval_context`.

It would be nice if we could split this API between local evaluation and remote evaluation because they do not actually have the same bounds. And local could work in any contexts without sync/async issues.

I'd be happy to discuss this and eventually work on this if this direction is accepted.

Start by reading the initialization path around flagsmith.options.enable_local_evaluation, update_environment, and eval_context. Compare the local-evaluation and server-side evaluation paths and define what synchronous and non-blocking APIs should expose; done means server-side evaluation no longer requires an eager fetch while local evaluation remains usable in sync and async contexts.

Written by the indexing model from the issue text.