Add IIpClient API for IpClient

The API will be used to interact with IpClient once it is moved to the
NetworkStack process.
This change keeps compatibility with the previous API so that clients
can be migrated independently.

Test: atest FrameworksNetTests NetworkStackTests
Bug: b/112869080
Change-Id: I57d8164afae849230bbc0e25f17859c0b22d0f29