Migrate apiclient to argh crate

[ytsssun]

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.