Support Synapse's (and technically Dendrite's) multi-key format (#513)

cmd/utilities/_common/signing_key_export.go

cmd/utilities/combine_signing_keys/main.go

cmd/utilities/generate_signing_key/main.go

homeserver_interop/any_server/signing_key.go

homeserver_interop/dendrite/signing_key.go

homeserver_interop/internal/signing_key_encode.go

homeserver_interop/mmr/signing_key.go

homeserver_interop/signing_key.go

homeserver_interop/synapse/signing_key.go

test/signing_anyserver_test.go

test/signing_dendrite_test.go

test/signing_mmr_test.go

test/signing_synapse_test.go