DVC-6643 - Update variable() to use new WASM variableForUser function

[chris-hoefgen]

• switched the SDK from getting all variables to using new variableForUser function in the WASM
• update logic to match what is done in Go/.Net/Ruby
• updated to latest WASM file
• Added data fixture files to load via class path instead of hardcoded variable
• Added large fixture file used for testing other SDKs like Go and DotNet
• Changed the unit tests to use an actual config web server to properly test initialization and variable evaluation

[chris-hoefgen]

This is probably a bit controversial but it got the job done. The more correct way is to build out a decent mock framework to serve the config data instead of using a real web server.

[chris-hoefgen]

we can probably replace this in a future PR by creating a way to inject an alternative DVCAPI instance into the constructor workflow for the DVCClient