proto: supporting zstd dictionary

DO NOT MERGE.

This is mainly served as a conversation starter for future dictionary
compression features.

Author: sluongng

proto/remote_execution.proto

@@ -2000,6 +2000,9 @@ message Compressor {
 
     // Brotli compression.
     BROTLI = 3;
+
+    // Zstandard compression with external dictionary support.
+    ZSTD_DICT = 4;
   }
 }
 
@@ -2036,6 +2039,18 @@ message CacheCapabilities {
   // [BatchUpdateBlobs][build.bazel.remote.execution.v2.ContentAddressableStorage.BatchUpdateBlobs]
   // requests.
   repeated Compressor.Value supported_batch_update_compressors = 7;
+
+  // A map of Compressor.Value to corresponding tree digest.
+  //
+  // The tree digest is expected to be shallow and contain only the dictionaries
+  // that may be used by the client to decompress the blobs received from the server.
+  //
+  // Dictionary files should have name corresponding to the dictionary's ID that is included
+  // in the header of each compressed blob.
+  //
+  // There SHOULD be a dictionary named "default" in the tree. It is to be used for compression
+  // when client uploads new blobs to the server.
+  map<string, Digest> compression_dictionary_trees = 8;
 }
 
 // Capabilities of the remote execution system.