Rename the package name to something like perun-node-api, node-api or similar

This issue comes out of the following discussion during review of #258.

"Does it make sense to rename the package name to something like perun-node-api, node-api or similar?
As this package name will be taken by frameworks to generate clients, the corresponding name will be taken for the package/module name. grpc is the technical framework and not the logical name.
This corresponds also to the other files in the folder."

_Originally posted by @danielksan81 in https://github.com/hyperledger-labs/perun-node/pull/258#discussion_r1271911452_