fix: Array type in JSONSchemaType should allow ReadonlyArray too

[t7yang]

What issue does this pull request resolve?
Currently developers can not use ReadonlyArray in JSONSchemaType like enum, items (for tuple), required, ...

This because in TypeScript, mutable array can assign to read-only array but read-only can not assign to mutable array.

Example

What changes did you make?
Convert every mutable array types to read-only array types in lib/types/json-schema.ts.

Is there anything that requires more attention while reviewing?
No.

[epoberezkin]

Just wondering - why do we need readonly arrays in schemas?

[t7yang]

Said data like this:

enum Foo {
  a = 'A',
  b = 'B'
}

// FOO can be the value of `enum` in a schema, and also expose to outside world to use
// for some reasons, we freeze this tuple to prevent it from mutation
export const FOO = Object.freeze([Foo.a, Foo.b]);

interface Bar {
  foo: Foo;
}

// currently TypeScript complain about can not assign readonly array to mutable array
const schema: JSONSchemaType<Bar> = {
  type: 'object',
  properties: {
    foo: { type: 'string', enum: FOO }
  },
  required: ['foo']
}

We do have other alternatives like create a new mutable tuple for enum to bypass this issue, but kind of cumbersome work.

The main idea is to enhance the capability of JSONSchemaType to accept both mutable and read-only array.
I think this can make DX better :)