Fixes linkerd#5575

Now that only viz makes use of the `SelfCheck` api, merged the `healthcheck.proto` into `viz.proto`.

Also removed the "checkRPC" functionality that was used for handling multiple API responses and was only used by `SelfCheck`, because the extra complexity was not granted. Revert to use the plain vanilla "check" by just concatenating error responses.

## Success Output

```bash
$ bin/linkerd viz check
...
linkerd-viz
-----------
...
√ viz extension self-check
```

## Failure Examples

Failure when viz fails to connect to the k8s api:
```bash
$ bin/linkerd viz check
...
linkerd-viz
-----------
...
× viz extension self-check
    Error calling the Kubernetes API: someerror
    see https://linkerd.io/checks/#l5d-api-control-api for hints

Status check results are ×
```

Failure when viz fails to connect to Prometheus:
```bash
$ bin/linkerd viz check
...
linkerd-viz
-----------
...
× viz extension self-check
    Error calling Prometheus from the control plane: someerror
    see https://linkerd.io/checks/#l5d-api-control-api for hints

Status check results are ×
```

Failure when viz fails to connect to both the k8s api and Prometheus:
```bash
$ bin/linkerd viz check
...
linkerd-viz
-----------
...
× viz extension self-check
    Error calling the Kubernetes API: someerror
    Error calling Prometheus from the control plane: someerror
    see https://linkerd.io/checks/#l5d-api-control-api for hints

Status check results are ×
```