[Fleet] Add validation to package policy input variables

[juliaElastic]

We had some occurrences when it was hard to debug what is wrong with the package policy create request.
Input variables have a type, and Fleet could do a validation that the value is valid for that type.
For example, integer type variable should have a valid integer value.

This would help debugging when the package policy creation fails on an invalid value.
E.g. trying to create a package policy with an input var max_agents of type integer would give a meaningful error when trying to use value 12,000

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

Resolved by #135257

Trying to add an invalid Fleet server policy with max_agent: notanumber result in

{"statusCode":400,"error":"Bad Request","message":"Package policy is invalid: inputs.fleet-server.vars.max_agents: Invalid integer"}%