Add group_member resource

[taiidani]

Fixes #44

Proposed Changes

• Adds a onepassword_group_member resource for managing group membership within a 1Password installation.
• This bumps the required version of the op client to 1.4.0 so that it receives the bug fixes applied to that version. In testing I also found that the automatic installation will not work on Mac due to their versions being distributed in a .pkg installer; I added error handling to more cleanly instruct the user in that case.

TODO

• [x] Import the changes from the terraform sdk v2 upgrade
• [x] Test a possible panic if someone attempts to import but doesn't use the hyphenated syntax
• [x] Update the required op version, as there is a bug fix inbound that only the newest client will contain.

[taiidani]

Currently waiting on a bug fix in the CLI where users who are not in the group being managed (e.g. administrators/owners) may encounter problems managing the members of that group.

See https://discussions.agilebits.com/discussion/comment/565941 for the conversation.

[taiidani]

Bug fix has been applied in 1.4.0. Updated this PR to have that as a required version.

[anasinnyk]

Thanks @taiidani