registry: Add get_target method

This adds a registry method to look up a target by name.

Change-Id: I55fb075e84014e9d5461fea06fe798eebeb58856
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/126223
Commit-Queue: Alexei Frolov <frolv@google.com>
Reviewed-by: Erik Gilling <konkers@google.com>