Consolidate click dependency and fix build-cache error handling
click>=7 introduces a breaking change where underscores in functions become dashes in the corresponding CLI command (so a function build_cache must be invoked via the CLI build-cache command).
- bump the minimum version for click dependency from >=5 to >=7
- also add an upper-bound to prevent similar issues from happening again (version 8.x was included in the range because it doesn't seem to include breaking changes that would affect s3keyring)
- update README to reflect the usage of dashes in CLI
command(s) - Fix build_cache error logic: currently if one of the secrets fail to be fetched/parsed (notably due to utf-8 decoding errors) either the execution crashes with an UnboundLocalError (if this occurs when processing the first secret) or the secret is assigned the value from the previous loop iteration (i.e the content of the secret processed just before the one failing)
- Add a --strict flag to raise an error instead of updating the s3 json cache file in case of errors (defaults to False). NOTE: the error is raised after looping through all the secrets so that all the failed secrets are printed before failing (which helps the user to troubleshoot all secrets instead of having to fix them one by one)