Move API key documents out of the security index

[ywangd]

With the increased usage of API keys (thousands and tens of thousands and frequent rekey?) as well as additional planned features like metadata (#48182), it may be beneficial to move API key documents into its own index, separated from the main security index. The other open issue about "last used" tracking (#54789) could also potential benefit from a separate API key index.

We do need to evaluate the situation and measure the performance carefully before making the decision. The issue is created to faciliate/kick off discussions.

[elasticmachine]

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