fix_: use proxy for mercuryo url signing

[dlipicar]

Cherrypicks #6966

[github-actions]

We require commits to follow the Conventional Commits, but with _ for non-breaking changes.
Please fix these commit messages:

fix: use proxy for mercuryo url signing

[status-im-auto]

Jenkins Builds

❔	Commit	#️⃣	Finished (UTC)	Duration	Platform	Result
✔️	f74794b	#1	2025-09-30 17:33:48	~3 min	macos	📦zip
✔️	f74794b	#1	2025-09-30 17:37:00	~6 min	windows	📦zip
✔️	f74794b	#1	2025-09-30 17:42:25	~12 min	linux	📦zip
✔️	f74794b	#1	2025-09-30 17:43:34	~13 min	tests-rpc	📄log
✔️	f74794b	#1	2025-09-30 17:58:53	~28 min	tests	📄log

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 30 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/10.30.x@4eca11e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
services/wallet/onramp/provider_mercuryo.go	6.25%	30 Missing ⚠️

❌ Your patch status has failed because the patch coverage (33.33%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@                Coverage Diff                 @@
##             release/10.30.x    #6971   +/-   ##
==================================================
  Coverage                   ?   58.57%           
==================================================
  Files                      ?      831           
  Lines                      ?   124624           
  Branches                   ?        0           
==================================================
  Hits                       ?    72993           
  Misses                     ?    44004           
  Partials                   ?     7627           

Flag	Coverage Δ
functional	30.42% <33.33%> (?)
unit	54.97% <24.44%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
api/default_networks.go	99.29% <100.00%> (ø)
api/defaults.go	76.04% <100.00%> (ø)
params/config.go	63.27% <ø> (ø)
protocol/requests/create_account.go	36.84% <ø> (ø)
services/wallet/service.go	82.96% <100.00%> (ø)
services/wallet/onramp/provider_mercuryo.go	41.52% <6.25%> (ø)

[dlipicar]

Posponed for now