Add additional included files in smithy-templates for smithy init

[hpmellema]

Description of changes:
This PR allows the inclusion of additional files and directories along with the template directory. The primary purpose of this is to allow templates to share common definitions of build files (i.e. gradlew, gradle/, gradle.properties, etc) without having to copy these files within each template directory.

For example, consider a gradle example for the smithy-quickstart-guide. The template directory for this template must contain:

my-template-repo/
├── smithy-templates.json
├── gradle
|        └── wrapper
│            ├── gradle-wrapper.jar
│            └── gradle-wrapper.properties
├── gradle.properties
├── gradlew
├── gradlew.bat
├── other/files
└─ getting-started-gradle
          ├── README.md
          ├── build.gradle.kts
          ├── gradle
          │           └── wrapper
          │              ├── gradle-wrapper.jar
          │              └── gradle-wrapper.properties
          ├── gradle.properties
          ├── gradlew
          ├── gradlew.bat
          ├── models
          │         └── weather.smithy
          └── smithy-build.json

In a repository with multiple examples, every single gradle example would need to copy the same gradle/ directory, gradlew gradel wrapper, etc.

Instead, we can include these files from outside the template directory from another location in the template-containing repository. For example, with this change, we could re-define the template in the smithy-template.json file as:

    "included-files-gradle": {
            "documentation": "Smithy Quickstart example weather service using the Smithy CLI with gradle files included.",
            "path": "getting-started/getting-started-gradle",
            "include": [
                "gradle",
                "gradlew",
                "gradle.properties"
            ]
        }

This would include the gradle wrapper files from the root of the template repo in the cloned template. Each new gradle template could just repeat this "include" definition without needing to copy the gradle wrapper definitions. So now the template directory would look like:

my-template-repo/
├── smithy-templates.json
├── gradle
│        └── wrapper
│            ├── gradle-wrapper.jar
│            └── gradle-wrapper.properties
├── gradle.properties
├── gradlew
├── gradlew.bat
├── other/files
└─ getting-started-gradle
        ├── README.md
        ├── build.gradle.kts
        ├── models
        │     └── weather.smithy
        └── smithy-build.json

As of right now all the files and directories are included at the root of the copied template directory

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