SignalR assets signing and publishing

Currently, there are two ways we are doing signing, because we have a legacy pipeline and a new unified build-based one. I want to make it do sign on demand before publishing. So, we can remove signing from legacy pipeline and from staging part of process for unified build pipeline.

There is a new way how we can sign them. I mean for GPG; there is a standard way which Ella created named ISigningService or something similar. We can implement that for these files to standardize the way how we do GPG signing across the pipeline.

## Goal

So, goal is to do the SignalR publishing only if assets are available (Not the case for SDK Only releases)

Steps:
- Check if we have assets to publish for SIgnalR - cancel the rest of the pipeline if not (SDK only release)
- Calculate checksums for the SignalR Maven assets
- Sign the assets and upload all the assets which will be published to Maven to the staging container
- Publish all the prepared SignalR assets together with signatures to Maven package

## Clean up

- Remove all previous signing phases. Legacy pipeline did that in prepare artifacts and unified-build based pipeline in staging publishing phase