Add a utility to JSON-format a CSR with build info

We need both the build fingerprint as well as the CSR when uploading
data to the APFE provisioning server. Add a utility function to format
the output as a JSON blob so that it may be easily collected in the
factory in a serialized data format, then later uploaded.

Test: libkeymint_remote_prov_support_test
Test: VtsAidlKeyMintTargetTest
Test: VtsHalRemotelyProvisionedComponentTargetTest
Bug: 191301285
Change-Id: I751c5461876d83251869539f1a395ba13cb5cf84