[SDTEST-3171] Quality of life improvements for Ruby and RSpec support

[anmarchenko]

What does this PR do?

Improves stability and usability of ddtest tool:

• now it automatically sets RUBYOPT variable to auto instrument Ruby projects with Datadog Test Optimization
• for test discovery mode, ddtest sets all needed environment variables automatically instead of assuming that user's environment already has them
• for RSpec test discovery ddtest now always uses bundle exec rspec command instead of bin/rspec - because knapsack_pro completely replaces bin/rspec which makes it unusable for our test discovery process
• as a small usability change ddtest always sets DD_CIVISIBILITY_ENABLED env variable, because it doesn't make sense to run ddtest without it
• ddtest also runs a very quick sanity check to verify that git executable is present and that we are running from a folder with git repository - and fails with error otherwise

On top of all that, this PR updates the dependencies to the latest versions.

Motivation

Early feedback from the first users uncovered some

How to test the change?

Manually tested using anmarchenko/forem project; otherwise unit tests are provided