wifi: add API for retrieving Hotspot 2.0 OSU providers

For an R2 Passpoint AP, there might be zero or more OSU providers
associated it.  Add an API to retrieve the associated OSU providers.

Bug: 62235301
Test: manual test by exercising this API in WifiTracker and verify
      the content of the OSU provider

Change-Id: I2e730f0b16d80e0b97b4ce7cbd086b606e71eb0c