Figure.colorbar: Plot gray scale or color scale bar

*This issue serves as the central place for discussing and tracking the implementation of the `colorbar` method in PyGMT. The issue will be closed when the initial implementation is complete. Progress is tracked at https://github.com/orgs/GenericMappingTools/projects/3.*

## Documentation

- GMT: https://docs.generic-mapping-tools.org/dev/colorbar.html
- GMT.jl: https://www.generic-mapping-tools.org/GMTjl_doc/documentation/modules/colorbar
- PyGMT: https://www.pygmt.org/dev/api/generated/pygmt.Figure.colorbar.html

## GMT Option Flags and Modifiers

☑️: *Implemented*; ⬜: *To be implemented/discussed*; ~~Strikethrough~~: *Won't implement*.

- [x] `-R`: `region`
- [x] `-J`: `projection`
- [x] `-B`: `label`/`unit`/`annot`/`tick`/`grid`/`annot_angle`/`annot_prefix`/`annot_unit`
- [x] `-C`: `cmap`
- [x] `-D`: `position`/`length`/`width`/`orientation`/`reverse`/...
- [x] `-F`: `box`
- [x] `-G`: `truncate`
- [ ] `-H`: newly added in GMT 6.7.0 #4783
- [X] `-I`: `shading`
- [ ] `-L`
- [x] `-M`: `monochrome` #4499
- [x] `-N`: `dpi` #4501
- [x] `-Q`: `log` #4244
- [ ] `-S`:
- [ ] `+a`: `annot_angle`
- [ ] `+c`
- [ ] `+n`
- [ ] `+r`
- [ ] `+s`
- [ ] `+x`: `label`
- [ ] `+y`: `unit`
- [ ] ~~`-U`~~: Use `Figure.timestamp` instead.
- [x] `-V`: `verbose`
- [x] `-W`: `scale` #4244
- [ ] ~~`-X`/`-Y`~~: Use `Figure.shift_origin` instead.
- [ ] `-Z`
- [x] `-c`: `panel`
- [x] `-p`: `perspective`
- [x] `-t`: `transparency`
- [ ] ~~`--PAR=value`~~: Use `pygmt.config` instead.

## Linked Pull Requests

- [x] Initial feature implementation
- [x] Add a tutorial or gallery example

## Related Issues and Discussions

*Add links to related wrapper discussions, API design threads, or upstream GMT changes here.*