featureDisallowIncoming is a new amendment that would allow users to opt-out of incoming Checks, Payment Channels, NFTokenOffers, and trust lines. This commit includes tests.
Adds four new AccountSet Flags:
1. asfDisallowIncomingNFTOffer
2. asfDisallowIncomingCheck
3. asfDisallowIncomingPayChan
4. asfDisallowIncomingTrustline