Return mutations and insertions as structured data

[JonasKellerer]

SILO now returns structured data for insertions and mutations. This should now also be returned by LAPIS. The SILO PR on insertions GenSpectrum/LAPIS-SILO#373 waits on this issue to be merged. The mutations PR is already merged (GenSpectrum/LAPIS-SILO#357).

The data looks like this:
Mutations:

{
      "count": 37,
      "mutation": "T19R",
      "mutationFrom": "T",
      "mutationTo": "R",
      "position": 19,
      "proportion": 0.38144329896907214,
      "sequenceName": "S"
 }

Insertions:

{
      "count": 5,
      "inserted_symbols": "EPE",
      "insertion": "ins_S:214:EPE",
      "position": 214,
      "sequenceName": "S"
    },

[Taepper]

inserted_symbols change to insertedSymbols for js conformity