feat: add sorted_set_fetch_by_index (momentohq#148)

* feat: add sorted_set_fetch_by_index

Adds a function to the simple cache client to fetch elements of a
sorted set selecting them by index (rank).

feat: Add CredentialProvider & support v1 tokens (momentohq#146)


Add the `CredentialProvider` construct that we have in other SDKs to parse auth tokens and get cache and control endpoints. This replaces the `EndpointResolver` and `jwt` modules.

Credential Providers can either be created from a string or an environment variable. They can optionally override the cache and control endpoints (or just one or the other).

```rust
    let credential_provider =
        CredentialProviderBuilder::new_from_environment_variable("TEST_AUTH_TOKEN")
            .build()
            .expect("TEST_AUTH_TOKEN must be set");

```

```rust
    let credential_provider =
        CredentialProviderBuilder::new_from_string("my_test_token")
            .with_cache_endpoint("cache.localhost")
            .with_control_endpoint("control.localhost")
            .build()
            .expect("could not parse token and endpoints")
```
then
```rust
 let mut momento = SimpleCacheClientBuilder::new(credential_provider, Duration::from_secs(30))?.build();
```

Co-authored-by: Kenny <[email protected]>

chore: derive Serialize and Deserialize for MomentoGenerateApiTokenRe…

…sponse (momentohq#137)

feat: add sorted set commands (momentohq#123)


Adds the sorted set commands for the Momento protocol to the SDK.

chore: Handle heartbeats in topics (momentohq#126)

This adds support for heartbeats that will be rolled out more broadly
soon. We'll need to be able to accept these messages and allow the
subscription to continue.

fix: update README to properly link examples directory (momentohq#121)

feat: remove redundant nested momento module (momentohq#93)

Before this commit, the auth module was in a nested momento module. So,
to access it, you would need to look in `momento::momento::auth`. This
is, frankly, weird and unnecessary. This commit removes the nested
momento module and drops the auth module into the crate root.

feat: add support for dictionary delete (momentohq#84)


Adds SDK support for the dictionary delete command.

feat: add dictionary fetch support to the Rust SDK (momentohq#82)

Adds support for the dictionary fetch command to the Rust SDK.

* Update src/simple_cache_client.rs
* fix test

chore: makes clippy check consistent for unwrap warning (momentohq#73)

Co-authored-by: Tyler Burdsall <[email protected]>