rgw: allow to set ssl options and ciphers for beast frontend

rgw: allow to set ssl options and ciphers for beast frontend

Two new conf keys are added for "beast" framework:

- ssl_options: a colon separated list of ssl context options,
  documented in boost's ssl::context_base;

- ssl_ciphers: a colon separated list of ciphers, documented
  in openssl's ciphers(1) manual.

Example:

  rgw frontends = beast ...  ssl_options=default_workarounds:no_tlsv1:no_tlsv1_1 ssl_ciphers=HIGH:!aNULL:!MD5

Fixes: https://tracker.ceph.com/issues/50932
Signed-off-by: Mykola Golub <mgolub@suse.com>