Merge branch '1556-key-manager-methods-for-pending-contacts' into 'master'

...api/src/main/java/org/briarproject/bramble/api/Bytes.java

.../java/org/briarproject/bramble/api/contact/ContactId.java

...rg/briarproject/bramble/api/contact/PendingContactId.java

...a/org/briarproject/bramble/api/nullsafety/NullSafety.java

...va/org/briarproject/bramble/api/transport/KeyManager.java

...org/briarproject/bramble/api/transport/StreamContext.java

...g/briarproject/bramble/api/transport/TransportKeySet.java

.../org/briarproject/bramble/contact/ContactManagerImpl.java

...va/org/briarproject/bramble/transport/KeyManagerImpl.java

...riarproject/bramble/transport/MutableTransportKeySet.java

...g/briarproject/bramble/transport/TransportKeyManager.java

...iarproject/bramble/transport/TransportKeyManagerImpl.java

.../briarproject/bramble/contact/ContactManagerImplTest.java

...rg/briarproject/bramble/transport/KeyManagerImplTest.java

...roject/bramble/transport/TransportKeyManagerImplTest.java

...rproject/briar/introduction/IntroduceeProtocolEngine.java