[code] deprecate user uploaded extensions

[akosyakov]

What it does

• fix code: deprecate user uploaded extensions #3308

It contains #4580 so should be merged afterwards.

Changes in Gitpod Code: gitpod-io/openvscode-server@19c309e

How to test

• Start a workspace: https://ak-deprecate-user-extensions.staging.gitpod-dev.com/#https://github.com/akosyakov/parcel-demo
• Create gitpod.yml with user uploaded extension records for instance:

vscode:
  extensions:
    - hangxingliu.vscode-nginx-conf-hint@0.1.0:UATTe2sTFfCYWQ3jw4IRsw==
    - zxh404.vscode-proto3@0.4.2:ZnPmyF/Pb8AIWeCqc83gPw==
    - ms-kubernetes-tools.vscode-kubernetes-tools@1.2.1:j58HdmA0K7d9a9sEkogZNw==
    - bajdzis.vscode-database@2.2.1:uXdjV53wtoTevFK6HOh3pQ==
    - hashicorp.terraform@2.1.1:QEP7gdWtMuY+j8RZ5OLDkA==
    - stkb.rewrap@1.13.0:yEx8nRSXlfXAHqsbkTJpdA==
    - golang.go@0.14.4:Z3iqHRuIxskc0nruY4MpEQ==
    - https://github.com/golang/vscode-go/releases/download/v0.26.0/go-0.26.0.vsix
    - https://github.com/golang/vscode-go/releases/download2/v0.26.0/go-0.26.222.vsix

• You should see a warning that user uploaded extensions are deprecated and a quick fix to resolve them all against Open VSX.
• Now add some invalid extensions in .gitpod.yml, you should get an error that an extension cannot be resolved.
  • Valid extension is either URL pointing to vsix file, just id, i.e. golang.go, or id@full-version, i.e. golang.go@0.14.4, everything else is invalid, i.e. golang.go@0

[JanKoehnlein]

LGTM. Awesome features!