Add configuration to modify downstream channel size

[ajohnstonTC]

What happened:
Write packets from the server to clients are dropped due to the downstream channel being bounded with a hardcoded size:

quilkin/src/cli/proxy/sessions.rs

Line 112 in 85652bb

let (tx, mut downstream_receiver) = mpsc::channel::<UpstreamChannelData>(5);

What you expected to happen:
Quilkin provides a configuration option to change the size of the downstream channel to mitigate dropped packets.

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Quilkin version: v0.8.0
• Execution environment (binary, container, etc): Container in Kubernetes
• Operating system: Amazon Linux 2
• Custom filters? (Yes/No - if so, what do they do?): No
• Log(s):

{"timestamp":"2024-03-20T18:49:02.030015Z","level":"ERROR","fields":{"message":"downstream channel full"},"target":"quilkin::cli::proxy::sessions","filename":"src/cli/proxy/sessions.rs"}
{"timestamp":"2024-03-20T18:49:02.030086Z","level":"ERROR","fields":{"message":"downstream channel full"},"target":"quilkin::cli::proxy::sessions","filename":"src/cli/proxy/sessions.rs"}
{"timestamp":"2024-03-20T18:49:02.030284Z","level":"ERROR","fields":{"message":"downstream channel full"},"target":"quilkin::cli::proxy::sessions","filename":"src/cli/proxy/sessions.rs"}
{"timestamp":"2024-03-20T18:49:02.030314Z","level":"ERROR","fields":{"message":"downstream channel full"},"target":"quilkin::cli::proxy::sessions","filename":"src/cli/proxy/sessions.rs"}
{"timestamp":"2024-03-20T18:49:02.030343Z","level":"ERROR","fields":{"message":"downstream channel full"},"target":"quilkin::cli::proxy::sessions","filename":"src/cli/proxy/sessions.rs"}