[I] Rust client should support comma-separated bootstrap servers [fluss-rust]

["luoyuxia (via GitHub)" <gi...@apache.org>]

luoyuxia opened a new issue, #585:
URL: https://github.com/apache/fluss-rust/issues/585

   ### Search before asking
   
   - [x] I searched in the [issues](https://github.com/apache/fluss-rust/issues) and found nothing similar.
   
   ### Please describe the bug 🐞
   
   The Rust client currently appears to support only one bootstrap endpoint, even though the public configuration is named `bootstrap_servers` / `bootstrap.servers`. A comma-separated value such as `127.0.0.1:9123,127.0.0.1:9124` should be accepted so users can configure multiple bootstrap nodes for high availability.
   
   Current behavior:
   
   1. Set the Rust client config to a comma-separated bootstrap list, for example:
   
   ```rust
   let mut config = Config::default();
   config.bootstrap_servers = "127.0.0.1:9123,127.0.0.1:9124".to_string();
   let conn = FlussConnection::create(config).await?;
   ```
   
   2. The client passes the whole string through `Metadata::new` / `Metadata::init_cluster`.
   3. `Metadata::parse_bootstrap` calls `to_socket_addrs()` on the complete string, so the comma-separated value is treated as a single socket address and fails to resolve.
   
   Expected behavior:
   
   The client should parse `bootstrap_servers` as a comma-separated list of `host:port` entries, trim whitespace, validate each endpoint, and use one reachable bootstrap endpoint to initialize metadata. Reinitialization should also keep using the configured bootstrap list instead of assuming a single address.
   
   Impact:
   
   Users cannot configure multiple bootstrap nodes/coordinators for HA or failover. This also makes the plural config name misleading.
   
   ### Solution
   
   Update the client bootstrap parsing to return a list of socket addresses rather than a single `SocketAddr`. During metadata initialization and reinitialization, try the parsed endpoints in a deterministic order, falling back to the next endpoint if one cannot be resolved or connected.
   
   Suggested test coverage:
   
   - comma-separated IPv4/hostname entries, e.g. `127.0.0.1:9123,localhost:9124`;
   - whitespace around entries;
   - IPv6 entries such as `[::1]:9123`;
   - invalid empty entries and invalid ports;
   - `reinit_cluster` uses the configured bootstrap list.
   
   ### Are you willing to submit a PR?
   
   - [ ] I'm willing to submit a PR!


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