osd: Use coroutines to perform synchronous reads in EC without blocking any threads

osd: Use coroutines to perform synchronous reads in EC without blocking any threads

Coroutines.h file added containing type and struct definitions.
Coroutine spawned in PrimaryLogPG::do_op if the message contains a CEPH_OSD_OP_CALL op.
ECBackend::objects_read_sync yields while running ECBackend::objects_read_async. Coroutine is resumed once the read is complete.
When a coroutine op is being processed, all other ops to the same PG are blocked.

Fixes: https://tracker.ceph.com/issues/74531
Signed-off-by: Matty Williams <Matty.Williams@ibm.com>