feat(backend): add custom webhook URL for POS

[sanducb]

Changes proposed in this pull request

This PR adds POS_WEBHOOK_SERVICE_URL env var and uses it when creating webhooks for incoming payment completions that were for card payments.

Context

Closes RAF-1145

Checklist

• Related issues linked using fixes #number
• Tests added/updated
• Make sure that all checks pass
• Bruno collection updated (if necessary)
• Documentation issue created with user-docs label (if necessary)
• OpenAPI specs updated (if necessary)

[github-actions]

🚀 Performance Test Results

Test Configuration:

• VUs: 4
• Duration: 1m0s

Test Metrics:

• Requests/s: 41.00
• Iterations/s: 13.68
• Failed Requests: 0.00% (0 of 2466)

📜 Logs

> [email protected] run-tests:testenv /home/runner/work/rafiki/rafiki/test/performance
> ./scripts/run-tests.sh -e test "-k" "-q" "--vus" "4" "--duration" "1m"

Cloud Nine GraphQL API is up: http://localhost:3101/graphql
Cloud Nine Wallet Address is up: http://localhost:3100/
Happy Life Bank Address is up: http://localhost:4100/
cloud-nine-wallet-test-backend already set
cloud-nine-wallet-test-auth already set
happy-life-bank-test-backend already set
happy-life-bank-test-auth already set
     data_received..................: 890 kB 15 kB/s
     data_sent......................: 1.9 MB 32 kB/s
     http_req_blocked...............: avg=6.73µs   min=2.29µs   med=5.27µs   max=1.01ms   p(90)=6.51µs   p(95)=7.11µs  
     http_req_connecting............: avg=319ns    min=0s       med=0s       max=244.86µs p(90)=0s       p(95)=0s      
     http_req_duration..............: avg=96.95ms  min=6.83ms   med=77.8ms   max=596.9ms  p(90)=171.05ms p(95)=186ms   
       { expected_response:true }...: avg=96.95ms  min=6.83ms   med=77.8ms   max=596.9ms  p(90)=171.05ms p(95)=186ms   
     http_req_failed................: 0.00%  ✓ 0         ✗ 2466
     http_req_receiving.............: avg=83.7µs   min=23.34µs  med=74.04µs  max=1.31ms   p(90)=111.19µs p(95)=135.79µs
     http_req_sending...............: avg=36.64µs  min=11.22µs  med=27.2µs   max=1.12ms   p(90)=40.54µs  p(95)=57.13µs 
     http_req_tls_handshaking.......: avg=0s       min=0s       med=0s       max=0s       p(90)=0s       p(95)=0s      
     http_req_waiting...............: avg=96.83ms  min=6.7ms    med=77.69ms  max=596.26ms p(90)=170.97ms p(95)=185.83ms
     http_reqs......................: 2466   40.998316/s
     iteration_duration.............: avg=292.11ms min=179.35ms med=280.42ms max=1.12s    p(90)=332.16ms p(95)=362.63ms
     iterations.....................: 823    13.682731/s
     vus............................: 4      min=4       max=4 
     vus_max........................: 4      min=4       max=4 

[mkurapov]

I would say, we can remove the fallback URL even. Might cause unexpected issues