Add convenience initializer for CustomAttribute

[fwcd]

This makes CustomAttributes constructible in a way similar to FunctionCallExprs:

// @Test
CustomAttribute("Test")

// @Test()
CustomAttribute("Test") {}

// @Test(a, label: b)
CustomAttribute("Test") {
  TupleExprElement(expression: "a")
  TupleExprElement(label: "label", expression: "b")
}

I have placed the convenience initializer under Sources/SwiftSyntaxBuilder/ConvenienceInitializers already to be consistent with #647.

[fwcd]

@swift-ci please test

[fwcd]

@swift-ci please test

[kimdv]

🚀