Address TODO comments across the code base

[shamblett]

There are a a few TODO comments building up across the code base, see below, these need to be either fixed or removed if no longer needed.

Found 17 TODO items in 6 files

coap_link_format.dart
(22, 6) // TODO(JKRhb): Revisit typing
(63, 6) // TODO(JKRhb): This needs to be a value between 1 and 4294967295.

integer_option.dart
(27, 8) // TODO(JKRhb): The handling of endianness should be revisited here.
(46, 8) // TODO(JKRhb): The handling of endianness should be revisited here.
(190, 4) // TODO(JKRhb): Is this really a class U option?
(228, 4) // TODO(JKRhb): Is this really a class E option?
(238, 4) // TODO(JKRhb): Is this really a class E option?

oscore_option.dart
(8, 4) // TODO(JKRhb): This currently can only contain encoded values and does not
(22, 6) // TODO(JKRhb): Maybe this can be done more elegantly

coap_client.dart
(40, 4) // FIXME: The name MatchETags might be a bit misleading, c.f. https://datatracker.ietf.org/doc/html/rfc7252#section-5.10.8.2

coap_media_type.dart
(336, 6) // TODO(JKRhb): Are there any uncovered cases?

coap_message.dart
(107, 10) // TODO(JKRhb): Revisit conditions for rejection
(238, 6) // TODO(JKRhb): Should rejections be included here as well?
(430, 6) // TODO(JKRhb): Apply proper percent-encoding
(469, 6) // TODO(JKRhb): Apply proper percent-encoding
(512, 6) // TODO(JKRhb): Apply proper percent-encoding
(560, 6) // TODO(JKRhb): Apply proper percent-encoding