Support multiline indexing statements

[thomasht86]

Example:

field title type array<string> {
        indexing {
            "en";
            index | summary;
        }
    }

which should be supported if the statement is a tuple.
And created like this:

Field(
    name="title",
    type="array<string>",
    indexing=('"en"', ["indexing", "summary"]),
)

Here are related vespa docs: https://docs.vespa.ai/en/reference/schema-reference.html#indexing.
We should keep current syntax, but also allow
Look at #1097 for a good idea on which files to change.
Be sure to test for quote escaping.
Add unit tests to test_package.py.
Update docstring (and doctest) and add example.