remote_exec.proto: support ZSTD with dictionary

Closes #272

build/bazel/remote/execution/v2/remote_execution.proto

@@ -1948,7 +1948,7 @@ message Compressor {
  // not need to advertise it.
  IDENTITY = 0;
 
- // Zstandard compression.
+ // Zstandard compression without dictionary.
  ZSTD = 1;
 
  // RFC 1951 Deflate. This format is identical to what is used by ZIP
@@ -1961,9 +1961,30 @@ message Compressor {
 
  // Brotli compression.
  BROTLI = 3;
+
+ // Zstandard compression with dictionary.
+ //
+ // When this is used, the server MUST advertise the dictionaries by
+ // including
+ // [ZstdDictionaryRegistry][build.bazel.remote.execution.v2.ZstdDictionaryRegistry]
+ // digest in CacheCapabilities.
+ ZSTD_DICT = 4;
  }
 }
 
+message ZstdDictionaryRegistry {
+ // Each file respresent a single Zstandard dictionary
+ // with name being the `dictId` that was added to the header
+ // of the compressed file.
+ //
+ // A special file with name `default` is used to represent the
+ // default dictionary that is used when no `dictId` is specified.
+ // Clients SHOULD prefer to use the default dictionary when possible.
+ //
+ // The `default` dictionary MUST be present.
+ repeated FileNode dictionaries = 1;
+}
+
 // Capabilities of the remote cache system.
 message CacheCapabilities {
  // All the digest functions supported by the remote cache.
@@ -1997,6 +2018,11 @@ message CacheCapabilities {
  // [BatchUpdateBlobs][build.bazel.remote.execution.v2.ContentAddressableStorage.BatchUpdateBlobs]
  // requests.
  repeated Compressor.Value supported_batch_update_compressors = 7;
+
+ // The digest of the
+ // [ZstdDictionaryRegistry][build.bazel.remote.execution.v2.ZstdDictionaryRegistry]
+ // that contains all the dictionaries supported by the remote cache.
+ Digest zstd_dictionary_registry = 8;
 }
 
 // Capabilities of the remote execution system.