V15 Metadata: Support accessing custom types

[jsdw]

V15 metadata introduces a new custom type map, something like BTreeMap<String, { type_id: u32, value: Vec<u8> }>.

Once this is stable, let's expose them; maybe with an API something like:

let custom_types = api.custom_types();

// Get some custom type "Foo", `None` if not found.
let ty = custom_types.get("Foo")?;

// Attempt to decode into a concrete type (Foo impls DecodeAsType):
// can use this to decode into `Value` too. `as_` because we have `as_event`,
// `as_extrinsic` or whatever. `decode_as_type` might be nicer.
let something: Foo = ty.as_type()?;

// Return the raw bytes:
let scale_bytes = ty.encoded();

// Return the raw type ID:
let ty_id = ty.type_id();