llext: Addressable ELF loading

[teburd]

Is your enhancement proposal related to a problem? Please describe.
Currently llext assumes a non-addressable source of an ELF, e.g. i2c flash, which may be copied from given a specific location. This causes a lot of unnecessary allocations for addressable ELF data.

Describe the solution you'd like
While sections themselves have different requirements and perhaps could be seen in #63535 for
getting elf metadata perhaps some function pointers would suffice.

struct llext_loader {
   int (*get_ehdr)(struct llext_loader *ldr, elf_ehdr_t **ehdr);
   int (*get_shdr)(struct llext_loader *ldr, size_t idx, elf_shdr_t **shdr);
}

This way each loader can decide how to best get a reference to some particular metadata rather than
forcing a file I/O like API on top of a buffer, we can assign the pointer value to where it is in the buffer. For non-addressable elf loaders custom logic can copy and cache and return a pointer as is done now.

Describe alternatives you've considered
A peek function like #62433 also works, but I think having metadata accessors is nicer for the elf portion and hopefully creates single functions for things like looking up a section or symbol name. I could be wrong though and need to experiment a bit.

[teburd]

@lyakh @pillo79

[teburd]

This has been solved with the the peek interface and WRITABLE kconfig