## Description 

This PR adds a new function `build_mainnet` corresponding to the mainnet
RPC node for the `SuiClientBuilder` type.

## Test plan 

How did you test the new or updated feature?

```rust
let sui_mainnet = SuiClientBuilder::default().build_mainnet().await?;
println!("Sui mainnet version: {}", sui_mainnet.api_version());
```

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## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [x] Rust SDK: Added a `build_mainnet` function to the `SuiClientBuilder`.
- [ ] REST API: