Add a Payment Flow Strategy object

[elia]

Summary

Ref #210

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

Merging #230 (69cb741) into master (f827aec) will increase coverage by 1.63%.
The diff coverage is 97.59%.

❗ Current head 69cb741 differs from pull request most recent head d470375. Consider uploading reports for the commit d470375 to get more accurate results

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
+ Coverage   96.39%   98.03%   +1.63%     
==========================================
  Files          23       24       +1     
  Lines         444      458      +14     
==========================================
+ Hits          428      449      +21     
+ Misses         16        9       -7     

Impacted Files	Coverage Δ
...ll/templates/config/initializers/solidus_stripe.rb	100.00% <ø> (ø)
app/models/solidus_stripe/setup_intent.rb	95.65% <85.71%> (-4.35%)	⬇️
lib/solidus_stripe/payment_flow_strategy.rb	96.96% <96.96%> (ø)
...p/controllers/solidus_stripe/intents_controller.rb	100.00% <100.00%> (+15.09%)	⬆️
app/models/solidus_stripe/gateway.rb	96.55% <100.00%> (+0.06%)	⬆️
app/models/solidus_stripe/payment_intent.rb	100.00% <100.00%> (ø)
app/models/solidus_stripe/payment_method.rb	100.00% <100.00%> (ø)
config/routes.rb	100.00% <100.00%> (ø)
...lates/app/views/checkouts/payment/_stripe.html.erb	100.00% <100.00%> (+4.16%)	⬆️
lib/solidus_stripe/configuration.rb	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[elia]

Superseded by #249