[kbn/server-route-repository] Make params optional for Zod schema with only optional keys

[miltonhultgren]

io-ts and zod have differences in how they usually define optional keys.

In io-ts, we usually define a union between a type and a partial, and if all keys are optional we only provide the partial type.
While in zod if all keys are optional, it's more common to simply put each key as optional while the wrapping object is not.

The way the type inference works in the route repository leads to this two cases having different results.
For io-ts, when it sees a partial, it marks the query/path/body and thus params object as fully optional, while for zod since query/path/body are not optional but all their keys are, the chain is still marked as required.

This PR aims to align these behaviours by checking if the zod schema only specifies optional field, and if it does it marks the query/path/body level as optional so that params also can become optional.

This way, when calling the API via the client, if all params are optional, the second options argument can be omitted.
Similarly, the types in the handler are marked correctly as possibly undefined to prevent bugs.

The PR also adds a little isHttpFetchError helper with slightly better type support than what the Core version has.

[obltmachine]

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[sorenlouv]

lgtm