Default connection to "AzureWebJobsStorage"

From @mjiderhamn in https://github.com/Microsoft/azure-maven-plugins/issues/269

Plugin name and version

azure-functions-maven-plugin 1.0.0-beta-2

Expected behavior

com.microsoft.azure.functions.annotation.*Trigger.connection has a default value of empty string. Documentation (albeit for C#) says "If you leave connection empty, the Functions runtime uses the default Storage connection string in the app setting that is named AzureWebJobsStorage."

Actual behavior

If connection annotation attribute is not provided, the following error occurs when executing azure-functions:package:

[ERROR] Failed to execute goal com.microsoft.azure:azure-functions-maven-plugin:1.0.0-beta-2:package (package-functions) on project serverless-azure: Storage binding (blob/queue/table) must have non-empty connection. Invalid storage binding found on method: se.jiderhamn.serverless.azure.AzureFunction.transformBlob -> [Help 1]

Start with the com.microsoft.azure.functions.annotation.*Trigger connection attributes and the azure-functions-maven-plugin package goal named in the report. Trace where an omitted connection is rejected and check whether that validation belongs in this library or the plugin. Done means a trigger without connection uses AzureWebJobsStorage and the relevant packaging test succeeds.