[Core feature] Expose ExecutionIdentity in flytekit

### Motivation: Why do you think this is important?

Exposed `ExecutionIdentity` in flytekit would be useful to e.g. load user-specific tokens or settings in individual tasks.

### Goal: What should the final outcome look like, ideally?

Provide `ExecutionIdentity` information from flyte to`flytekit.current_context()` in flytekit.

### Describe alternatives you've considered

There is no way how to get any user information in flytekit. The backend does not expose this as an environment variable for tasks (even though it's a K8s pod label) and the context (`flytekit.current_context()` or `FlyteContextManager.current_context()`) doesn’t contain this info.

### Propose: Link/Inline OR Additional context

_No response_

### Are you sure this issue hasn't been raised already?

- [x] Yes

### Have you read the Code of Conduct?

- [x] Yes