bluetooth: ots: reduce directory listing memory usage

Allocating a buffer to hold the entire directory listing consumes a very
large sum of memory when a large number of OTS objects are supported.

This implementation minimizes the memory footprint of the directory
listing by only allocating the minimum necessary buffer size to allow
for efficient over the air transfer. The contents of the transferred
buffer are built incrementally and upon request.

The directory listing manipulation upon object creation and deletion can
be removed since the directory listing content is never constructed
until it is requested.

Signed-off-by: Abe Kohandel <abe.kohandel@gmail.com>