Make SAPI endpoint config option name(s) clearer

We currently support:
- `metadata_api_endpoint` (file/kwarg) option and `DWAVE_METADATA_API_ENDPOINT` (env var)
- `endpoint` (file/kwarg) option and `DWAVE_API_ENDPOINT` (env var) for specifying SolverAPI (SAPI) endpoint - legacy names back from the era when cloud-client worked only with SAPI.

And after #569:
- `leap_api_endpoint` (file/kwarg) option and `DWAVE_LEAP_API_ENDPOINT` (env var).

For consistency, we should:
- add `solver_api_endpoint`/`DWAVE_SOLVER_API_ENDPOINT` options, and
- make `endpoint`/`DWAVE_API_ENDPOINT` aliases of these new SAPI endpoint options

Optionally:
- deprecate `endpoint`/`DWAVE_API_ENDPOINT` aliases