lib: Add support for stream buffer to expand

lib: Add support for stream buffer to expand

Issue:
 Currently, during encode time, if required memory is
 more than available space in stream buffer, stream buffer
 can't be expanded. This fix introduces new apis to support
 stream buffer expansion.

 Testing:
 Tested with zebra nexthop encoding with 512 nexthops, which triggers
 this new code changes, it works fine. Without fix, for same trigger
 it asserts.

 Signed-off-by: Soumya Roy <souroy@nvidia.com>