Add "primary output" to RequestMetadata

In Bazel, there are many cases where a combination of ` + - ` is not sufficient to identify an action.

For example, given a cc_library target with a lot of source files

```python
cc_library(
name = "foo",
srcs = [
"foo1.cc",
"foo2.cc",
"foo3.cc",
],
hdrs = ["foo.h"],
)
```

Then there will be 3 separate CcCompile actions with the identical combination above.
This is unfortunate because our `RequestMetadata` message is mapping to that exact combination and therefore, makes it challenging for servers to differentiate `ByteStream.Write` requests that were made for `CcCompile(foo1.cc)` and `CcCompile(foo2.cc)`.

The solution for this, unfortunately, is to add another field to RequestMetadata:

```protobuf
--- a/proto/remote_execution.proto
+++ b/proto/remote_execution.proto
@@ -2486,6 +2486,10 @@ message RequestMetadata {
// or equality across invocations, though some client tools may offer these guarantees.
string configuration_id = 7;

+ // An identifier for the expected set of outputs which are produced by this action.
+ // e.g. the first entry among the [Command][build.bazel.remote.execution.v2.Command]'s output_paths.
+ string primary_output = 8;
+
```

This way, we can avoid breaking server/client relying on existing fields.

Start in proto/remote_execution.proto at the RequestMetadata message and review the existing fields and compatibility comments. Add the proposed primary_output field with its documented meaning, then confirm the schema change preserves existing fields and numbering.

Written by the indexing model from the issue text.