Extensions: warn not fail on UnknownError during signature verification

[dtivel]

Resolve #164726.

This change does a few things:

1. Re-enables extension signature verification on Linux armhf.

2. Logs a warning (not an error) if extension signature verification fails with UnknownError.

   Example of verification success:

   2022-12-21 11:13:55.387 [info] Extension signature verification: ms-vscode.cpptools. Verification status: Verified.
   

   Example of verification failure with UnknownError:

   2022-12-21 11:10:57.744 [warning] Extension signature verification: ms-vscode.cpptools. Verification status: UnknownError.
   

3. Allows an extension install/update to proceed if extension signature verification failed with UnknownError.

4. Reports via telemetry if extension signature verification failed with UnknownError.

Note that this PR does not fix the underlying problem (TBD) causing UnknownError.

CC @isidorn, @sandy081, @joaomoreno

[sandy081]

@dtivel Thanks for the changes.

Added a small change to use enum for verification status instead of two different properties.

[sandy081]

Thanks for the changes