llext: add ELF inspection APIs

Add APIs to inspect the contents of an ELF file loaded as an extension.
This is useful for applications that need to access the contents of the
extension in a more fine-grained way than the existing LLEXT APIs.

Use of these APIs requires the 'keep_elf_data' option to be provided via
struct llext_load_param to the 'llext_load()' call.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>