cli: put collector credential in a dotfile

In #1019, we're looking at making it easy and fun to run simple aggregations with Divvi Up with a few command line invocations. One rough edge we have is collector credential handling. We instruct users to do divviup collector-credential generate --save, which will write out something like ./collector-credential-11.json, and then subsequently that same path is passed to divviup api-client collect --collector-credential-file.

It'd be nice if we instead defaulted to storing credentials in files under ~/.divviup/ (or whatever $HOME is on a given OS), akin to how aws-cli will store authentication tokens and config for which token goes with which account in ~/.aws/{config,credentials}.

This could break compatibility with the tool's existing interface (where --save writes the credential to $PWD) but I think there are probably few enough users of this tool out there that we can afford to make incompatible changes.

Start by reviewing the divviup collector-credential generate --save and divviup api-client collect --collector-credential-file entry points described in the issue. Determine how credential paths should be selected under the user’s home directory on each supported OS, and how existing path-based usage should be handled. Done means generated credentials are stored and subsequently discoverable under ~/.divviup/.