feat(proxy): abstract lambda runtime api proxy (#669)

# What?

Abstracts the concept of the `proxy` from the Lambda Web Adapter
implementation.
This will unlock the usage of ASM.

# How?

Using `axum` crate, we create a new server proxy with specific routes
from the Lambda Runtime API which we are interested in proxying.

# Motivation

ASM and [SVLS-6760](https://datadoghq.atlassian.net/browse/SVLS-6760)



[SVLS-6760]:
https://datadoghq.atlassian.net/browse/SVLS-6760?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: duncanista

bottlecap/Cargo.lock

@@ -60,6 +60,7 @@ datadog-trace-obfuscation = { git = "https://github.com/DataDog/libdatadog", rev
 dogstatsd = { git = "https://github.com/DataDog/serverless-components", rev = "3f3347790d4c7f3393589ef7efb7ccf22405a89a", default-features = false }
 datadog-trace-agent = { git = "https://github.com/DataDog/serverless-components", rev = "3f3347790d4c7f3393589ef7efb7ccf22405a89a" }
 datadog-fips = { git = "https://github.com/DataDog/serverless-components", rev = "3f3347790d4c7f3393589ef7efb7ccf22405a89a", default-features = false }
+axum = { version = "0.8.4", default-features = false, features = ["default"] }
 
 [dev-dependencies]
 figment = { version = "0.10", default-features = false, features = ["yaml", "env", "test"] }