Support for OCL_ICD_ASSUME_ICD_EXTENSION

ocl-icd provides an `OCL_ICD_ASSUME_ICD_EXTENSION` environment variable which tells it to skip the check for `cl_khr_icd` when loading ICDs. There are some users that are depending on this functionality (e.g. to target implementations that don't have a fully compliant ICD implementation) that are unable to use the Khronos ICD Loader.

This would be pretty straightforward to implement and I think is low risk (off by default, requires explicit opt-in). Would there be any objections adding this functionality here?

Start by tracing the ICD-loading path and the existing check for cl_khr_icd, then inspect how environment variables are handled in the loader. Done means OCL_ICD_ASSUME_ICD_EXTENSION explicitly bypasses that check while the default behavior remains unchanged; the issue does not name specific files or tests.

Written by the indexing model from the issue text.