Rename `swap-endpoint` to better name

[AuHau]

swap-endpoint represent Bee's blockchain connectivity and nowadays is not linked anymore only to SWAP protocol, but also postage-stamps, in close future to the storage loterry etc. It is quite confusing naming, so I would suggest renaming it to something like rpc-endpoint. It could be done with deprecation, eq. supporting for some time (until another breaking release?) both options with priority over the new name, communicating this to users and eventually removing swap-endpoint.