`aws_lib`: Support HTTP proxy configuration

[lukebakken]

Feature request

Support HTTP(S) proxy configuration for AWS API requests.

Reference

erlcloud supports proxies through its HTTP client abstraction - both lhttpc ({proxy, HttpProxy}) and hackney ({proxy, Proxy}, {proxy_auth, ProxyAuth}) variants carry proxy settings from aws_config.

Why this matters

Corporate and locked-down network environments frequently route all outbound HTTP through a proxy. The standard AWS SDKs honor HTTP_PROXY/HTTPS_PROXY/NO_PROXY environment variables. aws_lib currently connects directly via Gun with no proxy support.

Proposed behavior

1. Honor HTTPS_PROXY / HTTP_PROXY / NO_PROXY environment variables.
2. Allow explicit proxy configuration via aws_config().
3. Apply to both AWS API requests and (where appropriate) metadata requests - note IMDS at 169.254.169.254 and ECS at 169.254.170.2 should generally bypass the proxy (NO_PROXY semantics).

Depends on / relates to HTTP client abstraction - proxy support is cleanest to implement once the client is behind a single interface.

[lukebakken]

Related: #95