V15 Metadata: Support accessing custom types (#1106)

* work in progress

* add custom types access

* nit

* custom values client

* adjust light client

* adjust doc comments

* adjust book for custom values in code gen

* format and check docs

* use ignore in docs in book

subxt/src/metadata/metadata_type.rs

@@ -3,6 +3,7 @@
 // see LICENSE for license details.
 
 use crate::error::MetadataError;
+
 use std::sync::Arc;
 
 /// A cheaply clone-able representation of the runtime metadata received from a node.