Add support for comma-seperated list and union types in path and querystring params

[nhtruong]

We're working on describing OpenSearch API using Smithy. This API has a lot of comma-separated lists, and any unions (i.e. oneOf keyword in OpenAPI term) as schemas for Path and QueryString parameters. However Smithy currently doesn't allow:

• List as a data model for Path params
• Union as a data model for Path params
• Union as a data model for QueryString params

Is there a way to bypass these restrictions? If not, can we add support for this use case?

[JordonPhillips]

Can you give examples of how you would expect all of these to behave?

[nhtruong]

For example, we have the following schemas in OpenAPI:

   Indices:
      oneOf:
        - $ref: '#/components/schemas/IndexName'
        - type: array
          items:
            $ref: '#/components/schemas/Index
   IndexName:
      type: string
      pattern: pattern: '^[a-zA-Z0-9][a-zA-Z0-9_]*$'

And we have this operation DELETE /{index} where {index} has Indices as its schema. Right now there's no way for us to describe this schema properly as an untagged union. As a workaround, we've tried to describe this path param as an array of IndexName but Smithy doesn't allow Lists in path names either. So we would end up with a lot of duct-tape to describe the data model in Smithy:

xDataType("string | string[]")
xMember("IndexName")
String Indices