Make SAPI endpoint config option name(s) clearer

[randomir]

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