feature: extra map markers

[Paul-Bob]

Description

This PR enables the ability to add custom markers to a map view.

self.map_view = {
  # ...
  extra_markers: -> do
    [
      {
        latitude: 37.780411,
        longitude: -25.497047,
        label: "Açores",
        tooltip: "São Miguel",
        color: "#0F0"
      }
    ]
  end,
  # ...
}

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  • extra_markers docs.avohq.io#234
• I have added tests that prove my fix is effective or that my feature works

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[Paul-Bob]

Rename custom_markers to extra_markers

[github-actions]

This PR has been merged into main. The functionality will be available in the next release.

Please check the release guide for more information.