[config-schema] error when using custom validation function with oneOf / UnionType

[pgayvallet]

This test:

import { schema } from '..';

describe('foo', () => {
  it('bar', () => {
    const s = schema.oneOf([schema.string(), schema.number()], {
      validate: value => {
        return undefined;
      },
    });
    s.validate('a string');
  });
});

currently fails with:

TypeError: schema.custom is not a function
at new Type (packages/kbn-config-schema/src/types/type.ts:61:23)
at new UnionType (packages/kbn-config-schema/src/types/union_type.ts:29:5)
at Object.oneOf (packages/kbn-config-schema/src/index.ts:179:10)

seems to be caused by something around

kibana/packages/kbn-config-schema/src/types/type.ts

Lines 60 to 62 in c4ddd00

	if (options.validate) {
	schema = schema.custom(options.validate);
	}

UnionType is based on joi's alternatives type: AlternativesSchema which extends AnySchema.

However after a closer look, this custom definition is not in joi's package, but in our custom joi definition file:

kibana/packages/kbn-config-schema/types/joi.d.ts

Lines 44 to 46 in 066613e

	interface AnySchema {
	custom(validator: (value: any) => string | void): this;
	}

so it's very likely that custom is not actually present on all joi implementations of AnySchema and that our definition is wrong.

Maybe other types are also affected by this bug.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)