feat(cmd-api-server): add GetOpenApiSpecV1Endpoint (HTTP GET)

The new endpoint can serve up the OpenAPI specification file of the
API server via a get request without any parameters needed.

Under the hood it uses the re-usable common base endpoint class that
we've recently added to the core package.

The test case at this file path demonstrates how to use it via the API
client:
`packages/cactus-cmd-api-server/src/test/typescript/unit/get-open-api-spec-v1-endpoint.test.ts`

Signed-off-by: Peter Somogyvari <[email protected]>

feat(connector-besu): add GetOpenApiSpecV1Endpoint (HTTP GET)

The new endpoint can serve up the OpenAPI specification file of the
plugin via a get request without any parameters needed.

Under the hood it uses the re-usable common base endpoint class that
we've recently added to the core package.

The test case at this file path demonstrates how to use it via the API
client:
`packages/cactus-plugin-ledger-connector-besu/src/test/typescript/
unit/get-open-api-spec-v1-connector-besu.test.ts`

Signed-off-by: Peter Somogyvari <[email protected]>

feat(cactus-core): add GetOpenApiSpecV1EndpointBase<S, P> class

This is the pre-requisite to finishing the task at
https://github.com/hyperledger/cacti/issues/1877

Having this generic endpoint class available will allow us to send in
another commit which then uses it to create the Open API spec endpoints
in all the plugin classes.

Example usage of the generic class looks like this:

```typescript
import {
  GetOpenApiSpecV1EndpointBase,
  IGetOpenApiSpecV1EndpointBaseOptions,
} from "@hyperledger/cactus-core";

import { Checks, LogLevelDesc } from "@hyperledger/cactus-common";
import { IWebServiceEndpoint } from "@hyperledger/cactus-core-api";

import OAS from "../../json/openapi.json";

export const OasPathGetOpenApiSpecV1 =
  OAS.paths[
    "/api/v1/plugins/@hyperledger/cactus-plugin-ledger-connector-besu/get-open-api-spec"
  ];

export type OasPathTypeGetOpenApiSpecV1 = typeof OasPathGetOpenApiSpecV1;

export interface IGetOpenApiSpecV1EndpointOptions
  extends IGetOpenApiSpecV1EndpointBaseOptions<
    typeof OAS,
    OasPathTypeGetOpenApiSpecV1
  > {
  readonly logLevel?: LogLevelDesc;
}

export class GetOpenApiSpecV1Endpoint
  extends GetOpenApiSpecV1EndpointBase<typeof OAS, OasPathTypeGetOpenApiSpecV1>
  implements IWebServiceEndpoint
{
  public get className(): string {
    return GetOpenApiSpecV1Endpoint.CLASS_NAME;
  }

  constructor(public readonly options: IGetOpenApiSpecV1EndpointOptions) {
    super(options);
    const fnTag = `${this.className}#constructor()`;
    Checks.truthy(options, `${fnTag} arg options`);
  }
}
```

And the associated OpenAPI specification's paths entry:

```json
"/api/v1/plugins/@hyperledger/cactus-plugin-ledger-connector-besu/get-open-api-spec": {
  "get": {
    "x-hyperledger-cactus": {
      "http": {
        "verbLowerCase": "get",
        "path": "/api/v1/plugins/@hyperledger/cactus-plugin-ledger-connector-besu/get-open-api-spec"
      }
    },
    "operationId": "getOpenApiSpecV1",
    "summary": "Retrieves the .json file that contains the OpenAPI specification for the plugin.",
    "parameters": [],
    "responses": {
      "200": {
        "description": "OK",
        "content": {
          "application/json": {
            "schema": {
              "type": "string"
            }
          }
        }
      }
    }
  }
},
```

Signed-off-by: Peter Somogyvari <[email protected]>