Utilities to create TCKs for HLT1

The persistent format of TCKs in git repositories was defined as part of !3286 (merged). This MR adds the following to hlttck_cdb_listkeys:

• allow input to be read from stdin;
• allow import from JSON;
• allow export to JSON;
• write a manifest for a git repository;
• use nlohmann::json instead of custom to_json code;
• slightly change the format of git commit messages to contain only the metadata required to create a manifest;
• factor some of the code out into HltServicesLib so it can be used downstream to load configurations from git without running the executable;
• allow TCKs to be added to an existing repository.

The JSON that is used to import a configuration is expected to include a JSON manifest in the pre-existing format under the "manifest" key.

Needed for Allen!1149 (merged) and MooreOnline!212 (merged) Goes with Moore!2305 (closed)