Basic support of @deprecated trait in Smithy model (#1570)

* Add helper for creating deprecated attribute

* `Attribute.Custom.deprecated` is the main logic for building up
  `#[deprecated]` attribute
* `RustWriter.deprecatedShape` is the counterpart of `documentShape`,
  but we do not going to generalize it as what `documentShape` does.
  Deprecated is only for Rust code and probably won't be used in other
  output language.

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

* Test `@deprecated` trait for RustWriter

* Support `@deprecated` trait for StructureGenerator

* Support `@deprecated` trait for UnionGenerator

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

* Support `@deprecated` trait for EnumGenerator

* Support `@deprecated` trait for TopLevelErrorGenerator

* Support `@deprecated` trait for CombinedErrorGenerator

* Support `@deprecated` trait for ServerCombinedErrorGenerator

* Support `@deprecated` trait for FluentClient

* Support `@deprecated` trait for BuilderGenerator

* Cleanup leftover in test

* Use `dq()` helper method instead of escaping by hands

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

* Leverage Kotlin null safety well

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

* Allow `deprecated` rustc lint rule

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

* Allow deprecated in unit tests

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

* Leverage kotlin null safety check again

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

* changelog: Support @deprecated trait

Signed-off-by: Weihang Lo <whlo@amazon.co.uk>

Co-authored-by: Matteo Bigoi <1781140+crisidev@users.noreply.github.com>