Skip creating functions.metadata when source gen version of function metadata provider is enabled

[kshyju]

When source generated version of function metadata provider is enabled, we do not need to create the functions.metadata file.

Currently the build task is creating an file with empty array as content.

[satvu]

We will need to add a condition that checks the FunctionsEnableWorkerIndexing MSBuild property to the 4 GenerateFunctionMetadata tasks in our MSBuild targets file (example here).

However, this is complicated by the fact that this task also contains the generation of the WorkerExtensions.csproj file (see line here). We'll have to separate out these tasks first then add the conditional checks.

Changes pending internal discussion/design in case these generation tasks were coupled together for a specific reason.