Add option to disable authentication keys

[roccodev]

As mentioned in #804 (which this PR is also a workaround for), Velocity's aim is to support as many setups as possible.

With the 1.19.1 update, it is no longer possible to cancel or edit the order of incoming messages, while still keeping support for signed chat and player reporting.
However, Velocity currently lacks the option to forgo message signing (e.g., to freely cancel and transform everything into a system message) for 1.19.1 clients and servers.

This PR migrates the old force-key-authentication option into a three-variant enum (key-authentication-policy), with the default being the exact same behavior before this change. Additionally, if force-key-authentication was manually turned off, this falls back to the enabled state.

Disabling authentication keys is something the user must do manually via a config change, and it is discouraged in the option's comment.

[roccodev]

Sounds good!

[Strahilchu]

It should be encouraged and the default setting for sure!

[gamingcodedev]

bump