Add `expressions` in AlleleTranslator and CnvTranslator `_from*` methods

This
```python
allele_translator.translate_from("17-7667909-C-A")
```
Should return this, with the originating expression in the expressions:
```json
{
"id": "ga4gh:VA.GwrfsSJ88Aenbrr1RFgP2jJ_zZhVpm-1",
"type": "Allele",
"digest": "GwrfsSJ88Aenbrr1RFgP2jJ_zZhVpm-1",
"expressions": [
{
"syntax": "gnomad",
"value": "17-7667909-C-A"
}
],
"location": {
"id": "ga4gh:SL.0BtDFWQ3t1v02m-GcqUnzKf6kkLSK3C2",
"type": "SequenceLocation",
"digest": "0BtDFWQ3t1v02m-GcqUnzKf6kkLSK3C2",
"sequenceReference": {
"type": "SequenceReference",
"refgetAccession": "SQ.dLZ15tNO1Ur0IcGjwc3Sdi_0A6Yf4zm7"
},
"start": 7667908,
"end": 7667909
},
"state": {
"type": "LiteralSequenceExpression",
"sequence": "A"
}
}
```

This should be done for as many `_from*` methods as possible