Create BoundProvider, NeoFormatter, and TypedNeoFormatter (#4877)

Part of #1317

This PR adds `TypedNeoFormatter` and `NeoFormatter` that use a type parameter to handle all datetime formatting.

It also adds `BoundProvider` to the icu_provider crate.

Author: sffc

CHANGELOG.md

@@ -31,6 +31,7 @@
   - `icu_provider`
     - (Small breakage) `DataPayload::new_owned()` is no longer `const`, this was a mistake (https://github.com/unicode-org/icu4x/pull/4456)
     - Add `NeverMarker` to allow for DataProvider bounds that never return data (https://github.com/unicode-org/icu4x/issues/4186)
+    - Add `BoundProvider` to allow temporal separation between key and request (https://github.com/unicode-org/icu4x/pull/4877)
   - `icu_provider_blob`
     - Blob v2 no longer allocates (https://github.com/unicode-org/icu4x/pull/4383)
 - FFI: