Need more lenient SourceSpecificMulticastJoin

[matthughes]

SourceSpecificMulticastJoin correctly enforces that group IP is in the range 232/8 as defined by RFC 4607 ( https://datatracker.ietf.org/doc/html/rfc4607). However, in practice, values outside this range are often used. MulticastJoin is a sealed class so it cannot be extended outside of this library.

I propose we add a UnvalidatedSourceSpecificMulticastJoin or provide some way to bypass the range validation SourceSpecificMulticastJoin.

[mpilquist]

LenientSourceSpecificMulticastJoin makes sense to me, providing a source of type A and group of type Multicast.