RequestMetadata: Make ToolDetails a recursive structure

This adds a `wrapped_tool_details` field to the `ToolDetails` message,
which allows all tools involved in the sending and receipt of a request
to record their information in the request metadata.

For example, it allows tracking information about both the user-facing
client tool and a potential client-side proxy tool to be attached to the
request in a structured fashion.

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

@@ -2088,6 +2088,13 @@ message ToolDetails {
 
  // Version of the tool used for the request, e.g. 5.0.3.
  string tool_version = 2;
+
+ // Details of other tools involved in the request
+ // This recursive structure allows each tool involved in a request to
+ // have its name and version included in the request metadata.
+ // For example, a client-side proxy might include the client tool's
+ // initial ToolDetails here along with its own name and version.
+ ToolDetails wrapped_tool_details = 3;
 }
 
 // An optional Metadata to attach to any RPC request to tell the server about an