fix: HTTP/2 sends now wait up to 5 seconds for flow-control credit instead of returning send_buffer_full

[speeddragon]

Disclaimer: This PR code was generated by AI.

I'm trying to address #895. I'm having the same issue with ex_aws, and this PR modifies the request to wait until a specific time rather than returning {error, send_buffer_full}.

This change allows waiting for the buffer to clear instead of returning an error.

How to use it with ExAws:

  config :ex_aws, :hackney_opts,
    h2_send_timeout: 5_000

Or per request:

  ExAws.request(operation, http_opts: [h2_send_timeout: 10_000]) 

TODO

• Run rebar3 eunit
• Update documentation about the new property