Subrequest API requires data support and API updates.

Is your feature request related to a problem? Please describe.
During the development of a retry module gaps in the current subrequest API support were identified.

• need getters for request status and subrequest depth
• need to increment cycle count
• the flags argument is not supported
• the args argument is not supported
• need to pass custom data to subrequest
• do not automatically destroy subrequest body (allow caller to handle body lifetime)
• suggested: provide a basic subrequest example module

Describe the solution you'd like
Add additional API support. This work has mostly been complete here but needs to be separated from the Event timer API work and linting updates. See the src/http/flags.rs and src/http/request.rs files for salient changes.

Additional context
Since this work is mostly completed by @avahahn and @f5yacobucci please see them for unclear details. This issue is a placeholder tracking issue to break apart an existing PR into its component features.

Start by reading src/http/flags.rs and src/http/request.rs, then compare the existing ava-subreq-depth branch. Clarify the intended component boundaries with @avahahn or @f5yacobucci. Done means the subrequest API updates are separated from the Event timer and linting work, with the listed support gaps addressed.