Support for inserting additional subsampling in conformer encoder
# What does this PR do?

Added support for specifying an additional subsampling layer in conformer encoder. The user can specify the type and the position of the subsampling layer.

**Collection**: asr

# Changelog 
- New `SubsamplingReductionModule` class in `nemo/collections/asr/parts/submodules/subsampling.py`
- Updated the `ConformerEncoder` in `nemo/collections/asr/modules/conformer_encoder.py` to accept the new reduction parameters for adding a new subsampling layer in the desired position.
- Updated the _transducer-related configs_ with reduction parameters.

# 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)
- [x] 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

## Who can review?

Everyone in the NeMo ASR team.
