Allow adding metadata when creating an API key

[bytebilly]

Context

Users can create API keys in Stack Management > API keys, specifying the name, permissions, and the expiration date.
With Elasticsearch 7.13, now API keys can have metadata attached to them.

Proposal

Improve the Create API key panel to include metadata.
Since metadata may be arbitrary data, we could use an approach similar to what we already do for permissions, a textbox where users will enter their own values.

This is a possible mockup of the final result:

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[bytebilly]

cc @arisonl @cristina-eleonora

[legrego]

Related: #93820

[cristina-eleonora]

It looks good to me 👍🏻

[legrego]

@bytebilly I think it deserves a separate issue/story, but we should explore if/how we want to display this metadata when viewing API Keys (not just creating).

[bytebilly]

I think it deserves a separate issue/story, but we should explore if/how we want to display this metadata when viewing API Keys (not just creating).

Agreed, I would consider it in the scope of the general UX review of the API key table, along with other improvements.