Support serverParts in customMapSource

[nono303]

tested for kml overlay in GE and OpenLayers (/maps.json) in browser

<?xml version="1.0" encoding="UTF-8"?>
<customMapSource>
    <name><![CDATA[OpenTopoMap]]></name>
    <minZoom>5</minZoom>
    <maxZoom>20</maxZoom>
    <tileType>png</tileType>
    <tileUpdate>IfModifiedSince</tileUpdate>
    <ignoreErrors>true</ignoreErrors>
    <backgroundColor>#00000000</backgroundColor>
    <url><![CDATA[https://{$serverpart}.tile.opentopomap.org/{$z}/{$x}/{$y}.png]]></url>
    <serverParts>a b c</serverParts>
</customMapSource>

[grst]

I just enabled github actions for this repo -- could you please rebase on master and make sure that all tests pass?

(Test coverage in this package is rather low, but it's better than nothing...).

Other than that, let me know when you are ready for review ;)

[nono303]

if i did everything right... ready for review ^^

[grst]

LGTM, thanks!