Added support for GCP PubSub and Google Spanner database

[jhakp]

Extended the code and added support for GCP Pub/sub (message broker) and Google Spanner database.
Added a sample application "Sample.GcpPubSub.GoogleSpanner" to validate the changes. Open appsettings.json of "Sample.GcpPubSub.GoogleSpanner" and change the PubSub details and also change the spanner connection string to try out the changes.

[yang-xiaodong]

Hello,

As far as I know, Pub/Sub does not support multiple topics attached to one subscription, so we can't provide support for CAP group, Are you try to solve this?

[jhakp]

yes, Pub/Sub does not support multiple topics to one subscription. CAP group would not be supported but we can work without that.

[yang-xiaodong]

Since this is a core feature, it would be difficult for us to merge it into our main repository without support group feature, I would suggest that this could be written into our docs as a community supported extension and you as the maintainer, what do you think?

Community supported list docs:
https://cap.dotnetcore.xyz/user-guide/en/transport/general/#community-supported-transport-extensions

Thank you.

[jhakp]

I Yang, Google Pub/Sub would never support subscription group but that doesn't mean CAP cannot support GCP Pub/Sub. What Google recommends is to have only one subscription per Topic.
If you look at the code, we are registering only one subscriber. I don't think this would break any existing CAP implementation as the changes are local to the GCP Pub/Sub package.
I would sincerely like this change to be merged to master.

[praveenkrjha]

Any thoughts on merging this pull request to master?

[WeihanLi]

Suggested change

	public static CapOptions UseGooglePubSub(this CapOptions options,string projectId)
	public static CapOptions UseGooglePubSub(this CapOptions options, string projectId)