Add plugin_icon parameter to plugin description

[andreas-schultz]

• Support for an optional custom icon.
• Must be provided as data URL string.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
init.py	undefined	0	100%
dataintegration/init.py	0	0	100%
dataintegration/context.py	45	0	100%
dataintegration/description.py	138	9	93%	47 133 146 148 155 254 295 303-304
dataintegration/discovery.py	41	29	29%	26 40-45 47-51 53-55 69-70 72-81 87 89
dataintegration/entity.py	16	0	100%
dataintegration/plugins.py	23	5	78%	16 20 24 28 40
dataintegration/types.py	111	11	90%	76 88 93 102 134 137-139 148 151 254
dataintegration/utils.py	38	13	66%	21 23 27 37-42 45-46 49-50
dataintegration/parameter/init.py	0	0	100%
dataintegration/parameter/choice.py	23	1	96%	25
dataintegration/parameter/dataset.py	33	5	85%	29-30 33 53 56
dataintegration/parameter/graph.py	45	7	84%	42 68 71-73 83-84
dataintegration/parameter/multiline.py	4	4	0%	3 6 9-10
dataintegration/parameter/password.py	25	12	52%	11-12 16 31-34 36-37 42-44
TOTAL	543	96	82%

Tests	Skipped	Failures	Errors	Time
21	1 💤	0 ❌	0 🔥	12.604 ⏱