Add templates listing to smithy init command

[AndrewFossAWS]

Description of changes:

• Add templates listing to smithy init command
• Allow copying templates to nested output directory in smithy init command

Testing:

• smithy init --template quickstart-cli --output hello/world
• smithy init --template quickstart-cli --output ./hello/world
• smithy init --template quickstart-cli --output /Volume/workspace/hello/world
• smithy init --list

Output:

NAME                           DOCUMENTATION
----------------------------   ----------------------------------------------------------------------------------
custom-annotation-trait        Gradle project for creating a package for a custom annotation trait.              
quickstart-cli                 Smithy Quickstart example weather service using the Smithy CLI.                   
custom-string-trait            Gradle project for creating a package for a custom string trait.                  
common-shapes                  Common package for sharing shapes between multiple Smithy projects.               
custom-linter                  Gradle project for creating a package for a custom Smithy linter                  
filter-internal-projection     Projection that filters out internal shapes and traits.                           
codegen-starter                Directed-codegen plugin that generates simple POJOs.                              
common-linting-configuration   Common package for sharing linting configuration between multiple Smithy projects.
custom-validator               Gradle project for creating a package for a custom Smithy validator.              
custom-structure-trait         Gradle project for creating a package for a custom structure trait.               
quickstart-gradle              Smithy Quickstart example weather service using the Smithy-Gradle-Plugin.         
openapi-conversion             Generates an OpenAPI spec from a Smithy model

• smithy init -t invalid-template

Invalid template `invalid-template`. `Smithy-Examples` provides the following templates:

NAME                           DOCUMENTATION
----------------------------   ----------------------------------------------------------------------------------
custom-annotation-trait        Gradle project for creating a package for a custom annotation trait.              
quickstart-cli                 Smithy Quickstart example weather service using the Smithy CLI.                   
custom-string-trait            Gradle project for creating a package for a custom string trait.                  
common-shapes                  Common package for sharing shapes between multiple Smithy projects.               
custom-linter                  Gradle project for creating a package for a custom Smithy linter                  
filter-internal-projection     Projection that filters out internal shapes and traits.                           
codegen-starter                Directed-codegen plugin that generates simple POJOs.                              
common-linting-configuration   Common package for sharing linting configuration between multiple Smithy projects.
custom-validator               Gradle project for creating a package for a custom Smithy validator.              
custom-structure-trait         Gradle project for creating a package for a custom structure trait.               
quickstart-gradle              Smithy Quickstart example weather service using the Smithy-Gradle-Plugin.         
openapi-conversion             Generates an OpenAPI spec from a Smithy model 

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.