Codegen for custom values in metadata (#1117)

* 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

* work in progress

* add custom types access

* nit

* custom values client

* adjust light client

* codegen and validation

* adjust docs

* use ignore in docs in book

* change iter implementation

* use validation hash and other codegen changes

* add ui test for custom values codegen

* allow 'latest' metadata to be returned from the fallback code (#1127)

* nits

* fix validation check

* fix comments

* nits

---------

Co-authored-by: James Wilson <james@jsdw.me>

testing/ui-tests/src/lib.rs

@@ -11,6 +11,7 @@
 //! Use with `TRYBUILD=overwrite` after updating codebase (see `trybuild` docs for more details on that)
 //! to automatically regenerate `stderr` files, but don't forget to check that new files make sense.
 
+mod custom_values;
 mod dispatch_errors;
 mod storage;
 mod utils;
@@ -51,6 +52,12 @@ fn ui_tests() {
             .build(dispatch_errors::metadata_array_dispatch_error()),
     );
 
+    t.pass(
+        m.new_test_case()
+            .name("custom_values_foo")
+            .build(custom_values::metadata_custom_values_foo()),
+    );
+
     // Test retaining only specific pallets and ensure that works.
     for pallet in ["Babe", "Claims", "Grandpa", "Balances"] {
         let mut metadata = MetadataTestRunner::load_metadata();