Added support for SLD's by mounting styling files

[Daimanta]

Description

Adds support for SLD files by mounting the styling files in the pods for the legend-generator and mapserver. The styling can then be referenced by a STYLEITEM reference by using the standard styling.

Type of change

• New feature

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR