Replace - with _ when generating namespace for generated types

[kshyju]

Today. when you create a new project_(any project type_) in Visual studio with - in the name(Ex: "FOO-BAR"), The tooling will add an explicit root namespace entry to the .cpsoj file.

<RootNamespace>FOO_BAR</RootNamespace>

But we have customers reporting their existing projects with "-" in it running into invalid namespace name issue for the generated types when they upgraded to recent versions of worker sdk.

1. Microsoft.Azure.Functions.Worker.Sdk 1.15.0: issue with a RootNamespace containing a hyphen #1957
2. FunctionMetadataProviderGenerator code fails to build if project name has dash in name #2076

The fix is to always use the FunctionsGeneratedCodeNamespace msbuild proerty value. If user has not provided it, we will set the RootNamespace msbuild property value as the value of FunctionsGeneratedCodeNamespace.

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