Extension repository signing

[isidorn]

We now support repository signing. This means that every extension uploaded to the VS Marketplace from today will be code signed by the VS Marketplace.
Once a user installs a signed extension through VS Code's extensions view, VS Code will verify the signature and thus prove this extension is indeed coming from the VS Marketplace. The verification process is only done in VS Code Insiders. This issue is created so we enable it for VS Code stable.

There are no action items needs for extension authors.
Related extension author discussion microsoft/vscode-discussions#229
After we launch repository signing, we then plan to focus on publisher signing. Discussion here microsoft/vscode-discussions#137

To-do list:

• Fix Signature: UnknownError #164726 @dtivel
• Change default of feature flag in VS Code to be ON 🚀 @dtivel
• Document repository signing in VS Code release notes. Consider writing a blog post @isidorn @prashantvc
• 🏃 Run script to repository sign all existing extensions @nikhgup
• Remove feature flag on VS Code side @dtivel

[isidorn]

We have enable repository signing and today I plan to write release notes to cover this work.
Thus I am closing this issue as done.