Find a way to set cparams for the offsets of a super-chunk

[FrancescAlted]

Right now there is no way to set the compression properties (cparams) of the offsets chunk of a super-chunk. A possible solution is to create a defaults for that:

static const blosc2_cparams BLOSC2_CPARAMS_OFFSETS_DEFAULTS = {
        BLOSC_BLOSCLZ, 5, 0, 8, 1, 0, NULL,
        {0, 0, 0, 0, 0, BLOSC_SHUFFLE}, {0, 0, 0, 0, 0, 0},
        NULL, NULL, NULL};

and assign these cparams to a new attribute (say offsets_cparams) of the blosc2_storage structure. If the user wants to modify some, he just will need to modify the appropriate storage->offsets_cparams at their will.