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/custom_values/custom_value_address.rs

@@ -0,0 +1,21 @@
+use crate::dynamic::DecodedValueThunk;
+use crate::metadata::DecodeWithMetadata;
+
+/// This represents the address of a custom value in in the metadata.
+/// Anything, that implements the [CustomValueAddress] trait can be used, to fetch
+/// custom values from the metadata.
+pub trait CustomValueAddress {
+    /// The type of the custom value.
+    type Target: DecodeWithMetadata;
+
+    /// the name (key) by which the custom value can be accessed in the metadata.
+    fn name(&self) -> &str;
+}
+
+impl CustomValueAddress for str {
+    type Target = DecodedValueThunk;
+
+    fn name(&self) -> &str {
+        self
+    }
+}