#4199)

* refactor(core/exported): moved packet interfaces to packet.go

* feat(core/exported): 'PacketDataProvider' interface added

* feat(transfer): PacketDataProvider implemented

* feat(ica): implemented PacketDataProvider

* style(transfer_test, ica_test): improved test name

* docs(core.adr8): updated godocs

* style(ica_test): changed a variable name

* docs(core.adr8): added missing '.'

* imp(transfer): removed type assertion on jsonKey

* fix(transfer_test): removed unused test case parameter

* docs(transfer): updated godocs

* imp(ica): removed type assertion from 'GetCustomPacketData'

* imp(transfer_test): improved tests without type assertion

* imp(ica_test): improved tests without type assertion

* style(transfer_test): changed test case parameter name

(cherry picked from commit a0a6526)

# Conflicts:
#	modules/apps/27-interchain-accounts/types/packet.go
#	modules/apps/transfer/types/packet.go
#	modules/core/exported/channel.go