Migrate scaffolding form to use Scaffolding Service proxy via sidecar

[rohitsanj]

Summary of Changes

Any additional details or context that should be provided?

Pull request checklist

Please check if your PR fulfills the following (if applicable):

Tests

• Added new
• Updated existing
• Deleted existing

Other

• All new disposables (event listeners, views, channels, etc.) collected as for eventual cleanup?

• Does anything in this PR need to be mentioned in the user-facing CHANGELOG or README?
• Have you validated this change locally by packaging and installing the extension .vsix file?

  gulp clicktest

[rohitsanj]

I had to rename this from _enum (duplicated) -> enum manually. Which means the next time we run npx gulp apigen my changes will be lost. We'll need to file a bug report with openapi-generator to figure out what's going on here.

[rohitsanj]

Some notes:

1. Scaffolding Service rejects requests that have bad options (values that don't adhere to the specified pattern) -- I think we should relax this constraint at the service level. End-users should not need to provide perfect values in first pass. Update: We will not be able to relax the constraints since this is a requirement put forth by our security team.
2. Enforcing the pattern input attribute in the Scaffold HTML form means we'll have to be explicit about what the expected pattern is. Do users really care about what the expected regex pattern is for a given template option?
3. It will likely take a non-trivial amount of code changes to surface validation errors returned from the sidecar (via Scaffolding Service) into the Scaffold HTML form. We could start by simply showing a error notification that lists all the errors like "Option 'api_key' must match pattern 'a-A..'" and so on, but again, this offers poor UX.

[rohitsanj]

Decided to let vscode call scaffolding service directly without going through sidecar.