RequestMetadata: Make tool_details repeated

It is possible for more than one tool to be involved in the sending of a
gRPC request, for example when a client-side proxy is used it is
interesting to know the name and version of both the proxy and the
original client tool.

To facilitate this in RequestMetadata, this commit updates the
tool_details field to be `repeated`, allowing proxies and other related
tools to also be recorded in the metadata sent with requests.

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

@@ -2103,8 +2103,8 @@ message ToolDetails {
 // metadata, the user may ignore the base64 encoding and assume it is simply
 // serialized as a binary message.
 message RequestMetadata {
-  // The details for the tool invoking the requests.
-  ToolDetails tool_details = 1;
+  // The details for the tools involved in sending the requests.
+  repeated ToolDetails tool_details = 1;
 
   // An identifier that ties multiple requests to the same action.
   // For example, multiple requests to the CAS, Action Cache, and Execution