lockfiles: add `go.sum` parser
Created by: unknwon
This PR adds the go.sum
file parser with absolute minimum changes, i.e. methods for supporting Go proxy integration are not implemented (which should be addressed along with the other effort).
Validation checksum is not supported, but can be easily added in the future iterations.
Test plan
Unit tests
Part of https://github.com/sourcegraph/sourcegraph/issues/32658