ResourceMedia: support dynamic custom properties from HTL

followup from

it would be nice to have some kind of generic attribute map to pass custom properties which could be consumed in media processing steps or in the markup builder, e.g. lazy=true

because there is currently no syntax for defining map literals in the HTL spec (see https://github.com/adobe/htl-spec/issues/63), I suggest to define a prefix for request attribute names that should be considered as media properties and retrieve them manually from request and put them into MediaArgs properties.

For example: using prefix "mediaProp:"

the MediaArgs would have properties prop1='value1' and prop2='value2'