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/error/mod.rs

@@ -234,4 +234,7 @@ pub enum MetadataError {
     /// The generated interface used is not compatible with the node.
     #[error("The generated code is not compatible with the node")]
     IncompatibleCodegen,
+    /// Custom value not found.
+    #[error("Custom value with name {0} not found")]
+    CustomValueNameNotFound(String),
 }