erasure-code: add ceph_erasure_code debug command

It loads a designated erasure-code plugin and calls its
methods. It is convenient to figure out and tune the number of data
chunks, the size of an aligned chunk etc. For instance:

ceph_erasure_code \
      --parameter erasure-code-plugin=jerasure \
      --parameter erasure-code-directory=.libs \
      --parameter erasure-code-technique=reed_sol_van \
      --parameter erasure-code-k=2 \
      --parameter erasure-code-m=2 \
      --all

displays the chunk size when encoding an object of 1024 bytes.

get_chunk_size(1024)	512
get_data_chunk_count	2
get_chunk_count	4

Signed-off-by: Loic Dachary <loic@dachary.org>