add support an optional parameter in a query

[Damon-V79]

Hello!

We want to use optional parameters in an HTTP request, as it described here for pagination. "offset" and "limit" must be optional parameters.

This pull-request adds processing support for not required query parameter (i.e. /items?id=###).

Swagger:

paths:
  # dogs
  /dogs:
    get:
      operationId: getDogs
      produces:
        - application/json
      parameters:
        - name: limit
          in: query
          required: false
          type: integer
          format: int32
        - name: offset
          in: query
          required: false
          type: integer
          format: int32
      responses:
        200:
          schema:
            $ref: "#/definitions/Dogs"

  /just_test:
    get:
      operationId: justTest
      produces:
        - application/json
      parameters:
        - name: default
          in: query
          type: integer
          format: int32
        - name: required
          in: query
          required: true
          type: integer
          format: int32
        - name: optional
          in: query
          required: false
          type: integer
          format: int32
      responses:
        200:
          schema:
            $ref: "#/definitions/Dogs"

Old Scala code:

trait DogsApi extends Service {

    def getDogs(limit: Int, offset: Int): ServiceCall[akka.NotUsed, Dogs]
    def justTest(default: Int, required: Int, optional: Int): ServiceCall[akka.NotUsed, Dogs]

    final override def descriptor: Descriptor = {
        named("dogs").withCalls(
                restCall(Method.GET, "/api/v1.0/dogs?limit&offset", getDogs _),
                restCall(Method.GET, "/api/v1.0/just_test?default&optional&required", justTest _)
        ).withAutoAcl(true)
    }
}

New Scala code:

trait DogsApi extends Service {

    def getDogs(limit: Option[Int], offset: Option[Int]): ServiceCall[akka.NotUsed, Dogs]
    def justTest(default: Option[Int], required: Int, optional: Option[Int]): ServiceCall[akka.NotUsed, Dogs]

    final override def descriptor: Descriptor = {
        named("dogs").withCalls(
                restCall(Method.GET, "/api/v1.0/dogs?limit&offset", getDogs _),
                restCall(Method.GET, "/api/v1.0/just_test?default&optional&required", justTest _)
        ).withAutoAcl(true)
    }
}

Test project.