Add CompletableFuture Methods

[creatorfromhell]

This is mainly a placeholder issue to request that methods returning CompletableFutures be added to Vault. I plan to open a PR for this after PR #138 gets merged, but I wanted to open this as an advance notice so nothing is released as "vault2" when this PR would ultimately cause some incompatibility issues.

• Yes this is directly from my other issue but it ultimately is the same case, it will be added to the vault2 package. -

This is a pretty straightforward request, which is to add some CompletableFuture methods in order to allow for asynchronous programming inside the economy tree of Vault. This will allow economy developers to be able to predictively determine if the method call is sync or async without doing anything such as what was seen in issue #144 .

[cerealcable]

Just commenting that we should also have some initial design discussions here like discussed in #145.