llext: introduce 'enum llext_storage_type'

This patch introduces an 'enum llext_storage_type' that defines the
storage type of the ELF data that will be loaded:

- LLEXT_STORAGE_TEMPORARY: ELF data is only available during
  llext_load(); even when the loader supports directly accessing the
  memory via llext_peek(), the buffer contents will be discarded
  afterwards.
  LLEXT will allocate copies of all required data into its heap.

- LLEXT_STORAGE_PERSISTENT: ELF data is stored in a *read-only* buffer
  that is guaranteed to be always accessible for as long as the
  extension is loaded.
  LLEXT may directly reuse constant data from the buffer, but may still
  allocate copies if relocations need to be applied.

- LLEXT_STORAGE_WRITABLE: ELF data is stored in a *writable* memory
  buffer that is guaranteed to be always accessible for as long as the
  extension is loaded.
  LLEXT may freely modify and reuse data in the buffer; so, after the
  extension is unloaded, the contents should be re-initialized before a
  subsequent llext_load() call.

To keep full compatibility with existing code, the storage type of
LLEXT_BUF_LOADER is determined by the CONFIG_LLEXT_STORAGE_WRITABLE Kconfig
option between LLEXT_STORAGE_PERSISTENT and LLEXT_STORAGE_WRITABLE.

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