Add identifierIssuer support in to_rdf and API methods

[mielvds]

Fixes #136

Adds a identifierIssuer option to pass a custom identifierIssuer to:

• jsonld.flatten()
• jsonld.frame()
• jsonld.link() via its existing frame() delegation
• jsonld.to_rdf()

jsonld.normalize() deliberately removes identifierIssuer from its copied options, so it cannot be forwarded into to_rdf() or canon code.

Added focused tests for:

• to_rdf(), flatten() & frame() using custom issuer
• normalize() not leaking a supplied issuer

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	218	4	98%	27, 180, 565–566
context_resolver.py	103	8	92%	78, 137, 151, 197, 203–205, 229
iri_resolver.py	141	3	98%	311–312, 322
jsonld.py	2521	161	94%	260–264, 313, 350, 355, 365, 382–397, 433–434, 476, 484, 504, 512–513, 621–626, 709–710, 725–726, 789–794, 819–820, 835–836, 846–847, 873, 916, 926, 947–948, 991, 998, 1066, 1089, 1111–1113, 1123–1126, 1141, 1164, 1176–1179, 1275, 1287–1290, 1310, 1339, 1372, 1376, 1413, 1456, 1475–1478, 1491, 1540, 1643–1650, 1677–1679, 1705–1708, 1884, 1922, 2228, 2359, 2368–2375, 2469, 2496, 2520, 2528, 3119, 3164, 3183, 3248, 3301, 3304, 3367, 3452, 3486, 3494, 3772, 3777, 3779, 3964–3968, 4079, 4121, 4184, 4419, 4707, 4839, 4873, 4902–4905, 4931–4932, 5115, 5179, 5206, 5280, 5282, 5300, 5398, 5447, 5679, 5681, 5720, 5722, 5731, 5873, 6001, 6088, 6208–6222, 6284, 6286, 6354, 6567, 6573–6577, 6616, 6619, 6709–6710, 6719
lib/pyld/documentloader
aiohttp.py	71	22	69%	29–39, 68, 74, 86, 98–106, 118–121, 138, 153–156
file.py	36	1	97%	49
requests.py	52	5	90%	57, 71, 83–91
scheme_directed.py	15	1	93%	38
TOTAL	3492	205	94%

Tests	Skipped	Failures	Errors	Time
1738	37 💤	0 ❌	0 🔥	2m 57s ⏱️