CustomExtensions for File with TimestampPropertys can't handle datetime objects

[maybe-sybr]

STIX 2.1 File SCOs require that their extensions be considered when generating deterministic UUIDs, serialised following JSON canonicalisation rules. Defining a CustomExtension for Files that include a timestamp property results in exceptions being raised when that property is provided as a datetime object rather than a Zulu formatted datestamp string. The following snippet demonstrates the issue:

import datetime
import pprint

import stix2.v21
import stix2.properties

@stix2.v21.CustomExtension(stix2.v21.File, "x-xtime-ext", (
    ("xtime", stix2.properties.TimestampProperty(), ),
))
class XTimeExtension():
    pass

try:
    f = stix2.v21.File(
        name="foo",
        extensions={"x-xtime-ext": {"xtime": datetime.datetime.utcnow()}, },
    )
except Exception as exc:
    print(exc)
else:
    raise Exception("That should have failed...")

f = stix2.v21.File(
    name="foo",
    extensions={"x-xtime-ext": {"xtime": str(datetime.datetime.utcnow())}, },
)
s = f.serialize()
pprint.pprint(s)
stix2.parse(s)

f = stix2.parse("""
{"type": "file",
 "spec_version": "2.1",
 "name": "foo",
 "extensions": {
   "x-xtime-ext": {"xtime": "1970-01-01T00:00:00Z"}
 }
}
""")
print(f)

Presumably the fix would involve teaching _Observable._generate_id() to use a STIXJSONEncoder rather than just calling stix2.canonicalization.Canonicalize.canonicalization() which uses the default JSONEncoder implementation.