Implement additional CLI install/management commands

[theory]

Based on designs from #6, implement additional CLI tools, to make the client a full-featured client people enjoy using to manage all their extensions. Likely features:

• [ ] List extensions
• [ ] Search extensions
• [ ] Determine environment (Postgres version, OS, architecture)
• [ ] Resolve and install all extension, system, and library dependencies
• [ ] Install (or re-install) packaging registry packages
• [ ] Install relevant binaries from down stream packagers, where available
• [ ] Build from source for all supported pipelines (anything not already done for #12)
• [ ] Validate an extension has been installed
• [ ] Confirm successful installation
• [ ] CREATE EXTENSION et al. in one or more databases
• [ ] Report installed extensions and status (out of date, CVEs)
• [ ] Report extensions used in databases (versions, used/not used, out of date)
• [ ] Report dependencies
• [ ] Use private registries as well as the Root Registry
• [ ] Support all platforms supported by Postgres itself