Add bazel flag for max_cas_entry size.

Description of the feature request:

As of today, the max_cas_entry_size is declared at the server side. In the event that Bazel sends a write request containing a digest larger than the specified max_cas_entry_size, the remote cache throws an 'out_of_range' error. By introducing a Bazel flag for max_cas_entry_size, it becomes possible to validate it on the client side, potentially eliminating the need for a round trip to the server. This improvement has the potential to enhance build performance.

What underlying problem are you trying to solve with this feature?

Improved build performance with remote cache.