Improve Attach to Running Container support

[egamma]

Today "Attach" has limited flexibility and starts from scratch with each attach / container.

Improve this by:

• Remembering the last workspaceFolder, installed extensions and dynamically forwarded ports for each image.
• Associate a devcontainer.json with the image of a container and allow the user to edit it.

[Chuxel]

The second item on this list is related to #633 (if not a dupe). Also, FYI, #538 also requests the ability to specify an alternate user for the attach workflow which is a related request.

[chrmarti]

Also added a command to open any of the existing configurations.