Allow TypeScriptIntegrations to extend typescript-codegen settings

[everett1992]

TypeScriptIntegration should be able to define additional TypeScriptSettings config properties to read user configuration, without generating a warning. TypeScriptIntegrations are extensions for typescript-codegen. They may require user configuration, like the configuration that exists for protocol, package, packageManager, ect.

Integrations can access settings in smithy-build.json typescript-codegen with TypeScriptSettings#getPluginSettings() but if the settings include any properties that are not predefined by the ArtifactType smithy will print a warning:

WARNING: Expected an object with possible properties of package, packageDescription, packageJson, packageManager, packageVersion, private,
 protocol, service, targetNamespace, but found additional properties: someOtherProperty (N/A [0, 0])

smithy-typescript should allow integrations to define new configurations without warning. One option would be to let the integration push new properties into the artifactType config properties. Another would be to add a integrations object property that is an open bag of properties defined by integrations. There is already an open bag like this - packageJson.

[srchase]

I don't see a simple way to push new config keys that wouldn't be overly complicated. An open bag of properties to be defined by integrations makes sense and would be just as good of a solution.

I'll tag this as a feature request.