Add configurable gRPC message size limits for server

Problem

When using LookupSubjects with complex permission schemas involving recursive relations (e.g., group#downwards_member) and set operations, requests fail with:

rpc error: code = ResourceExhausted desc = grpc: received message after decompression larger than max 4194304

Why client-side options don't help

• Client-side grpc.MaxCallRecvMsgSize() only affects the client connection
• The limit is hit on internal dispatch calls where CollectingDispatchStream buffers results during Union/Intersection/Exclusion operations
• The server uses gRPC's default 4 MiB limit with no way to configure it

Use case

• Org with ~100k users
• Complex permission schema with recursive group membership (group#downwards_member)
• LookupSubjects to find all users with a permission on a resource

Proposed Solution

Add configurable gRPC message size limits to GRPCServerConfig:

• --grpc-max-recv-msg-size (default: 4194304)
• --grpc-max-send-msg-size (default: 4194304)

Since GRPCServerConfig is shared, this also enables:

• --dispatch-cluster-max-recv-msg-size
• --dispatch-cluster-max-send-msg-size

Implementation

I have a working implementation ready to submit as a PR if this approach is acceptable.

Changes:

• Add MaxRecvMsgSize and MaxSendMsgSize fields to GRPCServerConfig struct
• Register corresponding flags in RegisterGRPCServerFlags()
• Apply grpc.MaxRecvMsgSize() and grpc.MaxSendMsgSize() server options in Complete()