Implement helpers for the upcoming composition test

The https://github.com/crossplane/crossplane/blob/main/design/design-doc-developer-experience-tooling.md design doc describes our plans about the upcoming composition testing functionality. This is an umbrella ticket to track my work on implementing various helpers that can be used either as subcommands or as libraries, and will be needed for the upcoming test command. Many of those derive from [xprin-helpers](https://github.com/crossplane-contrib/xprin/blob/main/docs/xprin-helpers.md).

- [x] `xr generate` https://github.com/crossplane/cli/pull/13
- [x] `xr patch --xrd` https://github.com/crossplane/cli/pull/61
- [ ] `xr patch --add-field --delete-field --edit-field`
- [ ] `xr patch --add-connection-secret`
- [ ] `xr patch --gen-uid`