NormalizerWithAudio  - Allow plural Serial verbalizers
**Is your feature request related to a problem? Please describe.**

Pluralized verbalizers of Serial objects are not available in the results from NormalizerWithAudio.normalize_with_audio(), but a large number of plural serials exist in English eg "B-52s" should allow for a verbalizer of "B fifty twos" in addition to "B fifty two S".  The correctly pluralized serial should be discernible with the CER and audio transcript. 

These appear often in the financial, military, and other realms (think "401ks" "1099s" "W2s" etc).

**Describe the solution you'd like**

Add a parameter to the Serial FST. If it ends in "s" (or a plural form), offer a plural form of serial[-2] to the verbalizers as an option (and drop serial[-1]).

This may be non-trivial when serials end in a letter eg "401k" --> "four oh one kays" may be more useful than "four oh one ks"

**Describe alternatives you've considered**

The usual regex preprocessing which should be more efficiently implemented in FSTs?

**Additional context**

```python
from nemo_text_processing.text_normalization.normalize_with_audio import NormalizerWithAudio

audio_normalizer = NormalizerWithAudio(input_case='cased')

texts = [
    "Twelve B-17s were totally destroyed.",
    "8 401ks were emptied.",
]

for text in texts:
    normalized_texts = audio_normalizer.normalize_with_audio(text, verbose=False)
    print('\n'.join([x for x in normalized_texts]))

    assert "eight four oh one kays were emptied." in normalized_texts or "Twelve B seventeens were totally destroyed" in normalized_texts
```