[Test-Proxy] Support `debug` parameter from `common` tool templates

Add `debug` mode to `test-proxy` common build templates.

For users who want to set the loglevel for only the test proxy (and not globally for all .NET tooling), they will be unable to set the proper variable through variable setting. This is due to the translation that occurs when setting an AzDO build variable.

`Logging__LogLevel__Azure.Sdk.Tools.TestProxy` -> `(azdo translation layer setting environment variables)` -> becomes `Logging__LogLevel__Azure_Sdk_Tools_TestProxy`. TestProxy doesn't honor this value.

This situation would only affect the .NET team, as they would be the only ones where the globally verbose .NET logging would negatively affect the experience. However they start the proxy as a separate process in the testframework (where they have the control to set the environment variables/debug mode directly).

CC @billwert for FYI as our discussion brought this up