make Nominatim host and protocol configurable via search service options

[jwkaltz]

Description

Restructure nominatim URL construction in order to make hostname and protocol overrideable.
Add tests for Nominatim.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/geosolutions-it/MapStore2/blob/master/CONTRIBUTING.md - I hope so, I could not find the CONTRIBUTING file
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x", remove the others)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Issue

What is the current behavior?
The Nominatim geocoding endpoint is currently hardcoded as
https://nominatim.openstreetmap.org in web/client/api/nominatim.js.

This prevents users from configuring a custom Nominatim instance, which is required for:

• Air-gapped / offline deployments
• Private Nominatim instances for higher rate limits
• On-premise deployments where external internet access is restricted

What is the new behavior?
Read host and protocol from the service options object, falling back to the current defaults if not provided. This is fully backwards compatible — existing configurations continue to work unchanged.

Breaking change

Does this PR introduce a breaking change? (check one with "x", remove the other)

• Yes, and I documented them in migration notes
• No

Other useful information

After this change, there will be more options in localConfig.json or via overrideLocalConfig:

{
  "type": "nominatim",
  "priority": 5,
  "options": {
    "host": "my-nominatim.example.com",
    "protocol": "https",
    "limit": 10,
    "polygon_geojson": 1
  }
}

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