[jax:custom_partitioning] Make SdyShardingRule a user facing class.

Move the parsing of a sharding rule string to a free function
str_to_sdy_sharding_rule. Move the building of the MLIR sharding rule to a free
function sdy_sharding_rule_to_mlir.

PiperOrigin-RevId: 704818640

Author: bixia1