[IOPID-1181] Add email uniqueness verification FF

[shadowsheep1]

⚠️ Depends on pagopa/io-services-metadata#745 ⚠️

Short description

This PR adds the email uniqueness validation FF.

[codecov]

Codecov Report

Merging #329 (02015f4) into master (e795a59) will decrease coverage by 0.14%.
Report is 3 commits behind head on master.
The diff coverage is 64.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
- Coverage   68.00%   67.86%   -0.14%     
==========================================
  Files         149      154       +5     
  Lines        5113     5197      +84     
  Branches      599      599              
==========================================
+ Hits         3477     3527      +50     
- Misses       1635     1669      +34     
  Partials        1        1              

Files	Coverage Δ
src/features/wallet/index.ts	100.00% <100.00%> (ø)
src/features/wallet/payloads/paymentMethods.ts	100.00% <100.00%> (ø)
src/features/wallet/persistence/userWallet.ts	95.65% <100.00%> (ø)
src/features/wallet/routers/index.ts	100.00% <100.00%> (ø)
src/features/wallet/routers/router.ts	100.00% <100.00%> (ø)
src/features/wallet/utils/onboarding.ts	75.00% <100.00%> (ø)
src/features/wallet/utils/payment.ts	100.00% <100.00%> (ø)
src/payloads/backend.ts	100.00% <ø> (ø)
src/server.ts	100.00% <100.00%> (ø)
src/routers/public.ts	67.96% <88.88%> (-0.04%)	⬇️
... and 4 more

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[dpulls]

🎉 All dependencies have been resolved !

[Ladirico]

LGTM!