Swift: Add some tests and model SecKeyCopyExternalRepresentation

[geoffw0]

This PR does two things:

• adds some tests of things that look like encryption keys (and certificates) as sensitive data that I had lying around in a local commit. These aren't currently flagged because heuristics based on the word "key" tend to produce a lot of false positives - but they're important cases we should figure out at some point (there is already an issue tracking this).
• solve a small slice of the problem by creating an explicit model for SecKeyCopyExternalRepresentation(::), that turns a SecKey object into data (which could potentially be stored or transmitted). This is the first sensitive data source defined explicitly using models-as-data - so a bit of plumbing is involved as well.

[MathiasVP]

LGTM!