Allow User to "Bring Their Own" Runtime for the Language Server for VS Code Extension

[gfs]

Is your feature request related to a problem? Please describe.
As reported in #565 some users prefer not to have the .NET Install Tool for Authors obtain the .NET Runtime and may want to provide their own.

Describe the solution you'd like
A mechanism by which the user can specify a path to dotnet.exe that the VS Code extension will respect, and if present, not call dotnet.acquire.

Describe alternatives you've considered
We could alternately just try to run dotnet path\to\LanguageServer.dll and if that fails, acquire the Runtime as needed, but the server is implicitly started by the client so this may not be the best approach.

Additional context
This is a feature available in the VS Code extension for Bicep. https://github.com/Azure/bicep/blob/6fd52773948a97f11a776b9fbbf16fc03439af90/src/vscode-bicep/src/language/client.ts#L164-L172