[Form lib] Add option to indicate that a validator is asynchronous

[sebelga]

Consumers of the form lib can add synchronous and asynchronous validators to their field and it works "out of the box". This has a not optimal side effect: asynchronous HTTP request are called twice.

Indeed, the form lib will first try to run all the validators synchronously. If it detects that one validators returns a then, it knows that it is asynchronous. It then re-run all the validations asynchronously. Which trigger a second HTTP request.

We need to add an option to the validations (e.g. isAsync: true) which allows the consumer to tell beforehand that there is an asynchronous validation so the form lib can immediately await for the validations to run.

[elasticmachine]

Pinging @elastic/kibana-stack-management (Team:Stack Management)

[sebelga]

Closing. This has been added in #109233

Change: https://github.com/elastic/kibana/pull/109233/files#diff-3e75c35c5d016b55290a30604c278eaf90bbd5a78a26613dbed19f0abd59212fR256