EC: Enhance EC replication config parsing from string and enable validation of it

Currently ReplicationConfigValidator does not validate EC related configuration properly, as ECReplicationConfig is missing the toString method required by the validator.

Also at the moment ECReplicationConfig's String based constructor does enable weird configs like 2 data and 6 parity blocks with a chunk size of 177.
I think we should be and can be more prescriptive, and allow only the supported configs in EcReplicationConfig, with the supported data-parity pairs, and chunk size divisible by 1KiB, and maximize it at 1024KiB, which in a 10-4 setup already means 14MB buffered data on the client side. These might be adjusted at will, or later if we think it is better to have bigger sizes, but I think initially these are good boundaries.