xso-proc: Add support for the codec field on attribute meta

This allows a custom TextCodec to be used for encoding and decoding the
attribute’s value, instead of FromXmlText and AsOptionalXmlText.

xso/ChangeLog

@@ -29,6 +29,8 @@ Version NEXT:
         translated into a boolean (`#[xml(flag)]`).
       - Generic TextCodec implementation for all base64 engines provided by
         the base64 crate (if the `base64` feature is enabled).
+      - New `codec` field on `attribute` meta, to support decoding and
+        encoding using any `TextCodec`.
 
 Version 0.1.2:
 2024-07-26 Jonas Schäfer <jonas@zombofant.net>

xso/src/from_xml_doc.md

@@ -263,6 +263,7 @@ The following keys can be used inside the `#[xml(attribute(..))]` meta:
 | `name` | optional *string literal* or *path* | The name of the XML attribute to match. If it is a *path*, it must point at a `&'static NcNameStr`. |
 | `default` | *flag* | If present, an absent attribute will substitute the default value instead of raising an error. |
 | `type_` | *type* | Optional explicit type specification. Only allowed within `#[xml(extract(fields(..)))]`. |
+| `codec` | optional *expression* | [`TextCodec`] implementation which is used to encode or decode the field. |
 
 If the `name` key contains a namespace prefix, it must be one of the prefixes
 defined as built-in in the XML specifications. That prefix will then be
@@ -284,6 +285,9 @@ If `type_` is specified and the `text` meta is used within an
 `#[xml(extract(fields(..)))]` meta, the specified type is used instead of the
 field type on which the `extract` is declared.
 
+If `codec` is given, the given `codec` value must implement
+[`TextCodec<T>`][`TextCodec`] where `T` is the type of the field.
+
 #### Example
 
 ```rust