Add ability to change a plots wall (including merge plot walls)

[Joshiiiii]

A function in the API to change the plot's border to a different block.
Also, maybe a function to change the plot wall.

This could be useful to add the ability to the Claim Subcommand to change the plot border on claim.
Or also just to customize the plot border/wall.

No alternative methods

[jasonw4331]

The config option already exists to have it generate with a custom block. Any further modification will require a plugin similar to this one. If someone wants to PR the feature I would be willing to review and accept it.

[J1b1x]

A 100% adjustable with ingame Management System

https://www.youtube.com/watch?v=VBUBuQQzA9k

(not for merge walls, but normal plot walls)