ggml: Allow for `gguf_reader` implementation to be overridden

Build "build_test_b6337_1" branch

server : enable /slots by default and make it secure (ggml-org#15630)

* server : enable /slots by default and make it secure

ggml-ci

* server : fix tests to pass `--no-slots` when necessary

* server : extend /props with info about enabled endpoints

ggml: Allow for `gguf_reader` implementation to be overridden

.github: drop 20.04 builds

.github: add jetson target against jetpack

Enable GGML_BACKEND_DL

REVIEW: drop dl backend

Enable GGML_BACKEND_DL

.github: parameterise the architectures and versions