Ruby 3.0 keyword arguments compatibility (pass two)

[thbar]

This PR fixes Kiba ETL keyword arguments support for Ruby 2.7+ (and replaces #86 which is a previous attempt I wasn't completely happy with). See #85 for initial issue.

This works equally with Ruby 2.7.1 and current Ruby master (2.8.0-dev in rbenv, with ruby -v giving 3.0.0dev).

Decision

Special thanks to @eregon and @mame for their valuable input during the past few months.

I have decided for now to use ruby2_keywords like Rails does, which means:

• the impact should normally be inexistent for Ruby 2.7 (time will tell how this fares with the final version of Ruby 3).
• and in a way which doesn't require the ruby2_keywords gem

Relevant literature

• Separation of positional and keyword arguments in Ruby 3.0
• Rails forum "New 2.7/3.0 keyword argument pain point" discussion
• Initial kiba-common PR with discussion
• Examples in the Rails codebase

[thbar]

Tested fine on a reasonably complex ETL setup. I'll merge.