[Penpal] cleanup XCM config setup regarding assets

[clangenb]

Closes #7314 by implementing all the subtasks mentioned in #7314 (comment).

Changes

Essentially, the main driver of all changes is that we adjust the Penpal runtime as follows:

• Make the native token the base token for buying weight (before it was a hybrid set up, probably not 100% intentional).
• Merge the Assets and the ForeignAssets pallet into one pallet called Assets, as the local assets can also be identified with a location starting with parents: 0.

Test Changes

I tried to keep the changes minimal in the tests in order to not harm any previously established invariants. Hence, in most cases I just did:

• Add a PEN<>WND pool in order to be able to pay xcm execution fees in WND
• Replaced the Penpal's teleportable asset with it's new location based version.
• In very few cases, I switched from WND to PEN to make the tests easier, when I was sure that no invariants would be harmed.
• The rest should only be renamings.

[clangenb]

Wow, this turned out to be much more involved than expected, however, I think it is ready for review now. It is only the two xcm-dry-run tests that are still failing, where I would love to get some input.

@franciscoaguirre , @acatangiu tagging you as I believe this is your domain. ;)

[github-actions]

No issues found.

[clangenb]

/cmd prdoc --audience runtime_dev --bump minor

[clangenb]

/cmd label T10-tests T15-bridges T6-XCM