Migrate JsonLdProcessor to RDFLib

[mielvds]

This PR migrates JsonLdProcessor to rdflib and completes the internal migration to rdflib. It introduces a legacy mode to preserve backward-compatibility of the public API.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	278	49	82%	34, 45, 48–53, 67, 77–78, 507, 529, 589–594, 599–608, 612–636, 657–658
context_resolver.py	102	8	92%	70, 129, 143, 189, 195–197, 221
iri_resolver.py	141	3	98%	307–308, 318
jsonld.py	2418	180	93%	269–273, 354, 391, 396, 406, 423–438, 474–475, 517, 525, 545, 553–554, 662–667, 747–748, 763–764, 824–829, 854–855, 870–871, 881–882, 907–908, 954, 963–969, 976, 986, 1018, 1027, 1034, 1105, 1128, 1150–1152, 1162–1165, 1180, 1203, 1215–1218, 1310, 1322–1325, 1345, 1374, 1407, 1411, 1439–1444, 1457–1459, 1469–1471, 1514, 1532–1535, 1544, 1589, 1692–1699, 1726–1728, 1733, 1754–1757, 1933, 1954, 2382, 2391–2398, 2490, 2517, 2540, 2549, 3068, 3121, 3124, 3187, 3272, 3306, 3314, 3592, 3597, 3599, 3648, 3791–3793, 3902, 3965, 4185–4186, 4200, 4488, 4620, 4654, 4683–4686, 4712–4713, 4896, 4960, 4987, 5061, 5063, 5081, 5179, 5228, 5460, 5462, 5501, 5503, 5512, 5651, 5777, 5864, 5984–5998, 6057–6063, 6130, 6343, 6349–6353, 6392, 6395, 6485–6486, 6495
util.py	63	2	97%	41, 114
lib/pyld/documentloader
aiohttp.py	65	22	66%	28–38, 69, 75, 87, 99–107, 118–121, 138, 153–155
requests.py	41	5	88%	55, 69, 81–89
TOTAL	3143	269	91%

Tests	Skipped	Failures	Errors	Time
1678	57 💤	0 ❌	0 🔥	12.637s ⏱️