Add `synchronization` and `aggregation` args to get_variable(). These args...

Add `synchronization` and `aggregation` args to get_variable(). These args will be used for distributed variables.

Add Enum `VariableSynchronization` with values for `synchronization`: AUTO, UNREPLICATED, ON_WRITE, ON_READ

Add Enum `VariableAggregation` with values for `aggregation`: NONE, SUM, MEAN. Replace all the aggregation methods strings in distribution strategy to the enum values.

Update Mirrored strategy to use these parameters to decide on whether a variable should be Mirrored or TowerLocal.

Update different distribution strategy value types to use the `VariableAggregation` Enum

PiperOrigin-RevId: 202736077