Define Command Buffers in core UR

[martygrant]

A design suggestion was made in a comment in the the Native CPU command_buffer.cpp file in 27bbfcf by @uwedolinsky

Stub implementations of UR experimental feature command-buffers
Taken almost unchanged from another adapter. Perhaps going forward
these stubs could be defined in core UR as the default which would
reduce code duplication. Adapters could then "override" these defaults.

Requested by @EwanC in #1059 (comment) to open this up as an issue for discussion so it doesn't get lost. I have made an additional comment in the file to link to this issue while it is discussed.

[pbalcer]

You can already set functions to NULL in the adapter table. The loader will return UR_RESULT_ERROR_UNINITIALIZED (probably not the best choice). There's no need to create stub implementations.

What does need to exist in the adapter library are all the DDI symbols. In the case of Command Buffers, all you need is this:

UR_DLLEXPORT ur_result_t UR_APICALL urGetCommandBufferExpProcAddrTable(
    ur_api_version_t version, ///< [in] API version requested
    ur_command_buffer_exp_dditable_t
        *pDdiTable ///< [in,out] pointer to table of DDI function pointers
) {
  // pDdiTable is null-initialized

  return UR_RESULT_SUCCESS;
}

(which appears to be missing in the native cpu PR)

Theoretically it is possible to make the loader allow these symbols to be missing, but I don't believe that to be a good idea.