Allow reuse of payment sources via admin

[rainerdema]

Summary

• Fixes #280
• Fixes #267

ℹ️ Please refer to #280 for more details about specific implementation decisions.

• [x] Allow the admin to reuse existing payment sources by introducing a dedicated source form. This enables the reuse of payment sources within the user's wallet (preventing the use of payment methods that the user has removed from their wallet).
• [x] Re-introduce authorize and purchase gateway methods

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] 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[bot]]

Codecov Report

Merging #278 (33b0ad4) into master (1136a79) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   99.59%   99.62%   +0.03%     
==========================================
  Files          27       30       +3     
  Lines         489      540      +51     
==========================================
+ Hits          487      538      +51     
  Misses          2        2              

Impacted Files	Coverage Δ
app/models/solidus_stripe/gateway.rb	100.00% <100.00%> (ø)
app/models/solidus_stripe/payment_intent.rb	100.00% <100.00%> (ø)
app/models/solidus_stripe/payment_method.rb	100.00% <100.00%> (ø)
...spree/admin/payments/source_forms/_stripe.html.erb	100.00% <100.00%> (ø)
...nts/source_forms/existing_payment/_stripe.html.erb	100.00% <100.00%> (ø)
...ource_forms/existing_payment/stripe/_card.html.erb	100.00% <100.00%> (ø)
lib/solidus_stripe/testing_support/factories.rb	100.00% <100.00%> (ø)

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