Support custom parameter processors on LTI 1.3

[giovannicimolin]

Investigate if it's possible to support custom parameter processors on LTI 1.3.
Related discussion: #150 (comment)

[usmanpm]

Grooming update 11/14/25: This may not be a priority for the LTI board at the moment, so closing this for now, and we'll look into this again if this feature is prioritized.