Support proto as persistent worker protocol

Currently all swift actions running as persistent workers expect the worker to receive `WorkRequest` as JSON payload.

https://github.com/bazelbuild/rules_swift/blob/646d6d229edc0b22ea3b9dc891062de519d27d4c/swift/internal/actions.bzl#L203

This is however unsupported in certain remote execution service providers, such as [EngFlow](https://docs.engflow.com/re/client/remote-persistent-workers.html). The remote spawns in such environments will have `requires-worker-protocol` ignored even if it is supplied as execution requirements and hence, the worker will fail with

```
what(): [json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - invalid literal; last read: '�'
```

upon reading the first line of the `WorkRequest` since a proto payload is being sent instead.

This issue is therefore a feature request to support `proto` as worker protocol.

Read swift/internal/actions.bzl around line 203 first, then trace how persistent workers currently receive JSON WorkRequest payloads. Verify that proto is supported as a worker protocol in remote environments such as EngFlow and that the first request no longer fails with the reported JSON parse error.

Written by the indexing model from the issue text.