Discrepancy in json returned by fts `SNIPPET` function

When using the fts `SNIPPET` function the emulator's output schema diverges from what production spanner produces.

Specifically, the emulator Returns `start_position` and `end_position` as keys inside the highlights array (https://github.com/GoogleCloudPlatform/cloud-spanner-emulator/blob/master/backend/query/search/snippet_evaluator.cc#L59-L60), whereas production returns `begin` and `end`.

And it entirely omits the `source_begin` and `source_end` keys at the snippet level (https://github.com/GoogleCloudPlatform/cloud-spanner-emulator/blob/master/backend/query/search/snippet_evaluator.cc#L202)

Given a query like
```sql
SELECT
SNIPPET(content, "test"),
FROM
test_table
WHERE
SEARCH(content_tokens, "test")
```

Production spanner returns

```json
{
"snippets":[
{
"highlights":[
{
"begin":1,
"end":5
}
],
"snippet":"Test est",
"source_begin":1,
"source_end":9
}
]
}
```
This matches what is documented here https://docs.cloud.google.com/spanner/docs/reference/standard-sql/search_functions#snippet

The same query against the emulator returns
```json
{
"snippets":[
{
"highlights":[
{
"start_position":1,
"end_position":5
}
],
"snippet":"Test est"
}
]
}
```

Running Spanner Emulator Version: 1.5.54

Because of this discrepancy, application code and deserializers that work in production fail during local testing against the emulator unless environment-specific workarounds are added.

It would be nice if this could be fixed so the emulator returns the snippets using the same schema as production spanner.