Add type annotations to 'util'
Add type annotations to 'util'

Type annotating at least one module as a starting
point to enable static type checking.

Signed-off-by: Teodora Sechkova <e7a2b861183aa3bdb0c9acf2cae91c9cd14116e3@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib