[gh-actions] publishing nuget package on release

[springcomp]

Fixes #114.

This PR adds a new publish.yaml workflow that triggers on new releases.
The release MUST be associated with a tag having the following format v<M.m.n> (version number).

How it works

• The C# assemblies have a strong-name key which is encrypted and can only be decrypted by the publish.yaml workflow using the SN_PASSPHRASE, SN_SALT and SN_INIT repository secrets. That makes sure the resulting assemblies have the official JMESPath Community "identity" (kindly transferred from @jdevillard).
• In Debug mode, (PR) the strong-name key which is available in the repository is for test puposes only and never published to NuGet.

The resulting package is published to NuGet.org.

Note: this PR is a draft, pending:

• @jdevillard successfully transferring ownership of the package to the JMESPath Community NuGet organization.
• @jdevillard successfully setting repository secrets SN_PASSPHRASE, SN_SALT and SN_INIT.

[springcomp]

@StefH this is an initial workflow to publish NuGet packages from Release tags.

I have made extensive tests in a personal repository and hopefully there are no (or very few) bugs.
I cannot make any promises, though.

Please, review and merge.

Once that works, a new release to the project with name v<VERSION> where <VERSION> looks like M.n.p will trigger a new workflow and publish a new version of the package. I suggest starting next version from v1.0.400.