feat: support regapic LRO

[summer-ji-eng]

Enable using rest client to make long running operation call.

Example to use Rest SpeechClient to make LRO call, generator need to read speech_v1.yaml to construct httpRules which is array of `google.api.IHttpRules. Then we pass down the parameter to build OperationClient.

Here is an sample code in speech_client.ts

this.operationsClient = this._gaxModule
            .lro({
            auth: this.auth,
            grpc: 'grpc' in this._gaxGrpc ? this._gaxGrpc.grpc : undefined,
            protoJson: protoFilesRoot,
            httpRules: [
                {
                    selector: 'google.longrunning.Operations.GetOperation',
                    get: '/v1/operations/{name=**}'
                },
                {
                    selector: 'google.longrunning.Operations.ListOperations',
                    get: '/v1/operations',
                }
            ]

        })
            .operationsClient(opts);

[alexander-fenster]

I just hope the override code works :)