Expose one endpoint to enable access to mulitple engines

JSON body example

```
{
"engine": "nmrdb.org or nmrshift or nmrxiv-preduct-v1"
"structure": "
  CDK     09012310592D

  1  0  0  0  0  0  0  0  0  0999 V2000
    0.0000    0.0000    0.0000 C   0  0  0  0  0  0  0  0  0  0  0  0
M  END",
"options": {
"spectra" : ["proton", "carbon"]
}
}
```