Enum initialization bug

## Describe the bug

There needs to be updates to `from_proto` and/or initialization for enums, in addition to what was added to `to_proto` in https://github.com/caikit/caikit/pull/95

## Sample Code

Currently an example with the `stop_reason` (an enum), when initialized with the Python enum, rather than the value (int), does not have equivalent attributes after to and from proto-ing, i.e.
```
dummy_generated_response = dm.GeneratedResult(
generated_token_count=2, text="foo bar", stop_reason=dm.StopReason.TIME_LIMIT
)

new = dm.GeneratedResult.from_proto(dummy_generated_response.to_proto())
assert new.stop_reason == dm.StopReason.TIME_LIMIT.value # works
assert new.stop_reason == dummy_generated_response.stop_reason.value # works
assert new.stop_reason == dummy_generated_response.stop_reason # fails
```

With a value (int) initialized, the attributes are as expected:
```
dummy_generated_response = dm.GeneratedResult(
generated_token_count=2, text="foo bar", stop_reason=dm.StopReason.TIME_LIMIT.value
)
new = dm.GeneratedResult.from_proto(dummy_generated_response.to_proto())
assert new.stop_reason == dm.StopReason.TIME_LIMIT.value # works
assert new.stop_reason == dummy_generated_response.stop_reason # works
```

## Expected behavior

Attribute access should be consistent i.e.
```
assert new.stop_reason == dummy_generated_response.stop_reason
```

## Observed behavior

Failure for the above when object is initialized with enum