feat: make operationParameter optional within endpoint parameters

[dcroote]

Closes #100

[bbenligiray]

Looks good to me on a cursory glance but it would be nice for @metobom to also review

[metobom]

LGTM, works as expected with the below OIS.

{
  "apiSpecifications": {
    "components": {
      "securitySchemes": {
        "CoingeckoSecurityScheme1ApiKey": { "in": "query", "name": "x_cg_pro_api_key", "type": "apiKey" }
      }
    },
    "paths": {
      "/{apiPath}": {
        "get": {
          "parameters": [
            { "in": "path", "name": "apiPath" },
            { "in": "query", "name": "vs_currencies" },
            { "in": "query", "name": "ids" }
          ]
        }
      }
    },
    "security": {
      "CoingeckoSecurityScheme1ApiKey": []
    },
    "servers": [{ "url": "https://pro-api.coingecko.com/api/v3"}]
  },
  "endpoints": [
    {
      "fixedOperationParameters": [],
      "name": "feed",
      "operation": { "method": "get", "path": "/{apiPath}" },
      "parameters": [
        { "name": "name", "required": true },
        { "name": "apiPath", "operationParameter": { "in": "path", "name": "apiPath" }, "required": true },
        { "name": "vs_currencies", "operationParameter": { "in": "query", "name": "vs_currencies" }, "required": true },
        { "name": "ids", "operationParameter": {"in": "query", "name": "ids" }, "required": true }
      ],
      "preProcessingSpecifications": [{ "environment": "Node", "value": "...", "timeoutMs": 5000 }],
      "postProcessingSpecifications": [{ "environment": "Node", "value": "...", "timeoutMs": 5000 }],
      "reservedParameters": [
        { "name": "_type", "fixed": "int256" },
        { "name": "_path", "fixed": "value" },
        { "name": "_times", "fixed": "1000000000000000000" }
      ]
    }
  ],
  "oisFormat": "2.1.0",
  "title": "Coingecko",
  "version": "1.0.0"
}