[@kbn/config-schema] Improve extends and rename it to intersection

[maryam-saeidi]

🍒 Summary

At the moment, for intersection functionality (similar to what we have in io-ts and zod), we can use extends:

const a = schema.object({ a: schema.string() });
const bKeys = { b: schema.string() };
const b = schema.object(bKeys);

//  C = A & B (intersection of A and B)
const c = a.extends(bKeys);

The idea is to extend extends functionality to support multiple inputs (c = a.extends(b, d, e, f, g)) and use the name intersection to match other libraries for easier discoverability. (example usage in Kibana)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

This one may be a hard one, as intersections are not officially supported by joi: hapijs/joi#2495

[pgayvallet]

Actually, it seems supported via Joi.alternatives(...types).match('all')

[pgayvallet]

I don't think extend and intersection types are the exact same thing or should be using the same API "approach"

instead of

c = a.extends(b, d, e, f, g)

I think I'd rather expose a new intersection mixin and do

c = schema.intersection(a, c, d, e, f, g);

right?