Migrate apiclient to argh crate

**Summary**

Replace custom argument parsing in `apiclient` with `argh` to align with other Bottlerocket components (certdog, bloodhound, ghostdog, etc.).

**Current issues:**
- Custom parsing becomes increasingly complex as we introduce new subcommands and arguments, which could introduce maintenance burden
- Current parsing implementation has side effect (e.g. the parsing of "-h | --help" would indirectly trigger program::exit()) which makes it very hard to unit test.
- Inconsistent with rest of codebase where we use `argh` for the CLI.

Locate apiclient's custom argument-parsing implementation and compare the argh-based CLI patterns used by certdog, bloodhound, and ghostdog. Replace the custom parser while preserving the existing subcommands and arguments, then verify that help handling no longer triggers program exit indirectly and can be unit tested.

Written by the indexing model from the issue text.