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.

Start by tracing Bazel's remote-cache write path and the existing server-side max_cas_entry_size setting. Determine where a client flag would be defined and checked, then verify that oversized digests are rejected before the remote-cache request and that the behavior is covered by the relevant tests.

Written by the indexing model from the issue text.