Add support for data source instance

[dikhan]

Is your feature request related to a problem?

Currently data sources are supported for root level GET paths (https://github.com/dikhan/terraform-provider-openapi/blob/master/docs/how_to.md#terraform-data-source-compliant-requirements).

Describe the solution you'd like

As a service provider
I want to be able to expose as data resource instance paths (e,g: /v1/cdns/{id}) 
So users can retrieve individual resource instance info by providing the id in the data source configuration

Acceptance criteria

Scenario: fetching an already existing cdn using cdn data source instance
Given an already created cdn with id = 1234 and the swagger file below
When a data resource sl_cdn_v1_instance is configured with id = 1234 and terraform is executed
Then the resulted data source info should contain in the state both the id matching the input id from the user and the corresponding label for that given cdn id.

• Input swagger file

swagger: "2.0"
paths:
  /v1/cdns/{id}:
    get:
      summary: "Get cdn by id"
      parameters:
      - name: "id"
        in: "path"
        description: "The cdn id that needs to be fetched."
        required: true
        type: "string"
      responses:
        200:
          schema:
            $ref: "#/definitions/ContentDeliveryNetworkV1"
definitions:
  ContentDeliveryNetworkV1:
    type: object
    properties:
      id:
        type: string
        readOnly: true
      label:
        type: string

• Terraform configuration file using cdn data source instance

data "sl_cdn_v1_instance" "my_cdn" {
  id = "someID"
}

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

This ticket came out of realizing that some use cases may not necesarely need to depend on the list operation as supported in #5 ; but rather just retrieve an already existing resource instance by providing simply its id.

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