Add support for customHandler section in host.json

[pragnagopa]

This PR adds support to specify custom handler section in host.json as discussed in #6080

• Here is an example host.json that specifies workingDirectory

{
	"version": "2.0",
	"extensionBundle": {
		"id": "Microsoft.Azure.Functions.ExtensionBundle",
		"version": "[1.*, 2.0.0)"
	},
	"customHandler": {
		"description": {
			"defaultExecutablePath": "dotnet",
			"workingDirectory": "CSharpCustomHandlers/bin/Debug/netcoreapp2.2",
			"arguments": [ "CSharpCustomHandlers.dll" ]
		},
                "enableForwardingHttpRequest": true
	}
}

• Environment variables can be specified as part of any element in description following the format
  %EnvVarName%. Here is an example

{
	"version": "2.0",
	"customHandler": {
		"type": "http",
		"enableForwardingHttpRequest": false,
		"description": {
			"defaultExecutablePath": "%GoWorkerEnv%",
			"arguments": [
				"%FUNCTIONS_CUSTOMHANDLER_PORT%"
			]
		}
	}
}

• Each element in description can also be overridden via EnvVars/AppSettings. Here is an example of AppSetting / Environment variable name to overrride arguments element
  AzureFunctionsJobHost:customHandler:description:arguments

Fixes #5692, Fixes #5921, Fixes #5547, Fixes #6080, Fixes #5882, Fixes #5615

Note: httpWorker section will be deprecated sometime later. For now, it is still supported.

[yojagad]

[anthonychu]

Can we please hold off on this while we discuss #6020? Thanks.

[pragnagopa]

Can we please hold off on this while we discuss #6020? Thanks.

Yes. I will hold off. Added comments in referenced issue. This PR would still be needed to support env vars in the existing config.

[fabiocav]

Couple of comments from a brief look.

[pragnagopa]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[yojagad]

[pragnagopa]

@yojagad - can you please take quick a look again as I had to resolve few merge conflicts

[pragnagopa]

@fabiocav - can you please take a look at the config customhandler options ? It would be good to to get this PR in for the next release.

[yojagad]

That was the only comment I had. Rest looks good!