schema: Add string interpolation UAST type about sdk

[juanjux]

Python has two variants:

"some string {w} named interpolation".format(w='with')
# or
"some string {0} positional {1}".format('with', 'interpolation')
# or
"some string {} implicit positional {}".format('with', 'interpolation')
# this one is the joinedstr:
f"f-string {w} some interpolated variable"

Joined strings are the second ones. I've just noticed that the first one doesn't have an integration test so I'll make a PR for it.

The format ones get a pretty complicated AST: Expression->Call where the args are the {}'s for the second and third case or keywords for the third and the string itself is inside func.QualifiedIdentifier.identifiers[0].