refactor `StaticMap::fetchTile` to use DW HTTP client

[mprins]

refactor StaticMap::fetchTile to use dokuwiki\HTTP\HTTPClient or dokuwiki\HTTP\DokuHTTPClient for better proxy handling...

environment

• php version: 7.4
• dokuwiki version: master
• plugin version / date: master

expected behaviour

all proxy variables should be picked up

actual behaviour

proxies behavious is flakey as not all set options are picked up (if at all)

see also #5