[SPARK-52428] Add configurable gRPC connection and request timeouts

[rafafrdz]

Summary

• Add connect_timeout and request_timeout fields to ChannelBuilder
• Add connect_timeout(Duration) and request_timeout(Duration) builder methods to SparkSessionBuilder
• Apply timeouts to the tonic Endpoint before connecting

Previously, Channel::connect() had no timeout, meaning hung connections could block indefinitely. Now users can configure:

let spark = SparkSessionBuilder::remote("sc://host:15002")
    .connect_timeout(Duration::from_secs(10))
    .request_timeout(Duration::from_secs(30))
    .build()
    .await?;

Test plan

• cargo build passes
• cargo test unit tests pass
• cargo fmt -- --check passes