Handle Smithy 2.0 integer enum shapes

[DavidOgunsAWS]

IntEnum shapes have been added to Smithy 2.0. In order to support Smithy 2.0 models, changes are need in client and server side code generation.

• EnumGenerator invoked in CodegenVisitor needs to be extended substantially or a new class for IntEnums.
  • Also ServerEnumGenerator invoked in ServerCodegenVisitor needs similar support for integers

Note: looks like i32 should be the representation https://awslabs.github.io/smithy/2.0/spec/simple-types.html#integer

[jdisanti]

Refactoring the EnumGenerator to work off of enum shapes instead of traits is being tracked in https://github.com/awslabs/smithy-rs/issues/1671