feat: support custom ids and $defs from draft 2020-12

[cakemanny]

For #43.

This pull request adds further functionality of resolving references relative to their base URI as set by the $id keyword. (The "$id" keyword, in draft-07 and in draft 2020-12).

It also expands the existing support of #/definitions/A style references, with #/$defs/A style references as found in draft 2020-12. (Schema Re-Use With "$defs", in draft 2020-12).

Since the json-schema 2020-12 spec is quite large and complicated it would be easy to get carried away with supporting various cases, I instead tried to be rather pragmatic. As has been the case so far, only subschemas in definitions or $defs are able to be referenced and so only they have their $id processed.

---

There are some features of how URI references are relative that could be implemented but I've chosen not to for now.

For example, given this following schema

{
    "$id": "https://example.com/root.json",
    "$defs": {
        "B": {
            "$id": "other.json"
        }
    }
}

We are not supporting

• "$ref": "https://example.com/other.json"
• nor "$ref": "root.json"

Both of which would be expected to resolve in a fully compliant implementation.

And with the following schema document

{
    "$id": "https://example.com/root.json",
    "$defs": {
        "B": {
            "$id": "https://example.com/other.json"
        }
    }
}

we are not supporting "$ref": "other.json".

---

Another caveat to point out is, due to how schemars loads $defs into definitions, it means that we treat #/$defs/A the same as #/definitions/A.
i.e. this schema is invalid but we'd still end up processing it

{
    "properties": {
        "hello": {"$ref": "#/definitions/A"}
    }
    "$defs": {
        "A": {"type": "string"}
    }
}

---

Looking forward to feedback and am happy to adapt if there's anything that not quite right or if you feel something is missing.

[untitaker]

lgtm please fix lints/formatting and we can merge

[cakemanny]

ok, formatting fixed 👍

[untitaker]

ty!