Complete rewrite of pcustom command (#794)

* Entirely rewrote the `pcustom` functionality to make it usable from other plugins (see #778)

* - applied PR comments
 - added cache

* Apply suggestions from @Grazfather

Co-authored-by: Grazfather <grazfather@gmail.com>

* Changed signature of `apply_at`

* Revert "Apply suggestions from @Grazfather"

This reverts commit 3948328.

* linting + formatting

* last fix

* making `Modules` and `Module` inherit `dict` so we can use getitem and iteration from it

* removed unused var in `PCustomListCommand`

* Apply suggestions from code review

Co-authored-by: theguy147 <37738506+theguy147@users.noreply.github.com>

* - added error message in `pcustom show` when not found
- passing a reference to `ExternalStructureLoader` to modules and structures for nested structure to work properly

* - added `pcustom list` test

* - added test for `pcustom show` with and without address

* Update gef.py

Co-authored-by: Grazfather <grazfather@gmail.com>

* - `pcustom` prints error message if struct name not found
- added CI tests  for error messages

Co-authored-by: Grazfather <grazfather@gmail.com>
Co-authored-by: theguy147 <37738506+theguy147@users.noreply.github.com>