Make gli an optional dependency

[corsonknowles]

Closes #601, implementing option (1) as agreed there.

gli was a runtime dependency, but only the slack command-line client (bin/slack) uses it. require 'slack-ruby-client' never loads it. Library-only users therefore installed gli for nothing, and with it a hard dependency on ostruct (GLI::Options < OpenStruct), which Ruby 4.0 no longer ships as a default gem.

• slack-ruby-client.gemspec: drops gli. The runtime dependencies are now faraday, faraday-mashify, faraday-multipart, hashie and logger.
• bin/slack: requires gli itself. When gli is missing it exits with status 1: "The slack command-line client requires the gli gem. Add gem 'gli' to your Gemfile, or run gem install gli."
• Gemfile: adds gli to the :test group, because spec/slack/slack_spec.rb runs bin/slack.
• README / UPGRADING / CHANGELOG: the Command-Line Client section now says to install gli, and UPGRADING has an entry for CLI users. Both are under 3.3.0 (Next), and Slack::VERSION is bumped to 3.3.0.

Tests: a new spec runs bin/slack with --disable-gems and no inherited Bundler environment, so require 'gli' raises LoadError on every Ruby, and asserts the exit status and the message. An earlier version stubbed Kernel#require instead. That didn't take on 2.7, 3.0, 3.1, 3.3 or JRuby, where another require wrapper runs first. The new spec also passes on Ruby 2.6. bundle exec rspec gives 615 examples, 0 failures, and RuboCop reports 75 files with no offenses (Ruby 3.4.11). After require 'slack-ruby-client', neither gli nor ostruct is loaded.

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[dblock]

Fix CI, increment version to 3.3.0 since we're making a feature change.

[dblock]

Thanks! Merging. I'll cut a release.

[dblock]

Thanks! Merging. I'll cut a release.

Made some other changes, so will wait a few days before cutting a release, appreciate any testing on master if you have time @corsonknowles

[corsonknowles]

Thanks @dblock !

We built the 3.3.0 gem and installed it into an empty GEM_HOME: it brings in no gli or ostruct, the library loads, with_response/with_error return separate client copies, and slack prints the install hint until gli is installed, then makes real requests.

I also pinned our main Rails app to 093baf4, coming from 3.1.0, and its full RSpec suite passed: 360,000 examples, no failures and no retries. gli and ostruct drop out of our lockfile.

Only found at best, 1 very minor item: #603 changes user-visible exception message text (e.g. TooManyRequestsError is now "retry after N seconds"), so it might be worth calling that out in the CHANGELOG or UPGRADING for anyone matching on error messages. We were not.

In any case, thumbs up from me.

[josecolella]

Thanks for merging this, @dblock! Would you be open to cutting 3.3.0 soon? We now flag ostruct in our lockfiles, and slack-ruby-client → gli is one of the paths we'd like to drop with a plain version bump.