Build GHfVS and Essentials extension

This key is for identity not security.

Signed an unsigned assemblies can behave differently. It's important
that they're consistently signed when testing.

Users can set this up once and won't need to modify the
ApiClientConfiguration_User.cs file in their working directory.

Ensure that build will pick up changes to GitHubVS_ClientId.

Remove dependency on private script submodule

Use a matrix build strategy.

Create the VSIX directly in the ArtifactStagingDirectory.

Sign the VSIX

Don't sign VSIX on Azure Pipelines PR builds

…e IGitActionsExt2.InitializeOrPushRepositoryToGitService()

…ionsExt2 under 16.0

Change GitHub.TeamFoundation.16 version of TeamExplorerServices

Prevent error when filenames have braces in them

Update Octokit.GraphQL schema

Fix detecting GitHub Enterprise Server behind a HAProxy

…nuget.config

Remove https://dotnet.myget.org/F/dotnet-core/api/v3/index.json from nuget.config