[ASR] Transcribe for multi-channel signals

# What does this PR do ?

This PR adds the following optional parameters to ASR transcription:
- select a single channel from multi-channel input
- override the default audio key (`"audio_filepath"`) when loading paths from a manifest

**Collection**: ASR

# Changelog 
- Added optional `channel_selector` parameter to `TranscriptionConfig`
- Added optional `audio_key` parameter to `TranscriptionConfig`
- Propagated the two parameters to `transcribe` and `transcribe_utils`

# Usage

Transcription config can be updated, for example as

```python
cfg.channel_selector = 0 # Evaluate on the first channel
cfg.audio_key = 'alternative_audio_filepath' # Load audio from an alternative filepath provided in the manifest
```

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
