Allow selection of alternate devcontainer.json from UI

[dmckernanacv]

Is your feature request related to a problem? Currently the only way to pick which devcontainer to use is via the CLI. This would allow the UI to detect all devcontainer configurations in a workspace and allow the user to choose which one to build.

Which solution do you suggest? When devcontainer.json detection happens, look for all possible devcontainer.json files and if there is more than one, ask the user which one they would like to apply to the workspace (and possibly optionally set it as default for the future)

Which alternative solutions exist? Currently this can be done manually via the CLI with the --devcontainer-path flag, however there is no way to supply this in the UI.

Additional context