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/book/usage/custom_values.rs

@@ -0,0 +1,54 @@
+// Copyright 2019-2023 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+//! # Custom Values
+//!
+//! Substrate-based chains can expose custom values in their metadata.
+//! Each of these values:
+//!
+//! - can be accessed by a unique __name__.
+//! - refers to a concrete __type__ stored in the metadata.
+//! - contains a scale encoded __value__ of that type.
+//!
+//! ## Getting a custom value
+//!
+//! Custom values can be accessed via a [`CustomValuesClient`](crate::custom_values::CustomValuesClient).
+//! The client exposes an `at` function by which a custom value can be fetched, given an address to this custom value.
+//! An address can be as simple as the aforementioned __name__ as a [str]. This will return a dynamic value, that you can manually decode into the type you want.
+//! Suppose, the custom types contain a value of type `Foo` under the name `"foo"` you can access it like in this example:
+//!
+//! ```rust,ignore
+//! use subxt::{OnlineClient, PolkadotConfig, ext::{codec::Decode, scale_decode::DecodeAsType}};
+//!
+//! #[derive(Decode, DecodeAsType, Debug)]
+//! struct Foo {
+//!     n: u8,
+//!     b: bool,
+//! }
+//!
+//! let api = OnlineClient::<PolkadotConfig>::new().await?;
+//! let custom_value_client = api.custom_values();
+//! let foo_dynamic = custom_value_client.at("foo")?;
+//! let foo: Foo = foo_dynamic.as_type()?;
+//!
+//! ```
+//!
+//! Alternatively we also provide a statically generated api for custom values:
+//!
+//! ```rust,ignore
+//! #[subxt::subxt(runtime_metadata_path = "some_metadata.scale")]
+//! pub mod interface {}
+//!
+//! let static_address = interface::custom().foo();
+//!
+//! let api = OnlineClient::<PolkadotConfig>::new().await?;
+//! let custom_value_client = api.custom_values();
+//!
+//! // Now the `at()` function already decodes the value into the Foo type:
+//! let foo = custom_value_client.at(&static_address)?;
+//! ```         
+//!
+//! Note: Names of custom values are converted to __snake_case__ to produce a valid function name during code generation.
+//! If there are multiple values where the names would be equal when converted to __snake_case__, functions might not be statically generated for some of them, because of naming conflicts.
+//! Make sure names in the custom values of your metadata differ significantly.