Add support for filtered vector search

[yoziru]

This PR adds support for metadata filters in vector search
Filters are based on MongoDB's query operators since some vector dbs already use this approach. For the first step, only string filtering is supported:

• $eq
• $ne
• $and
• $or

Float operators ($gte, $lte, ..) are for a later PR

Example simple

{
    "collection": "grafana.promql.templates",
    "query": "count over time",
    "filter": {
        "metric_type": {
            "$eq": "gauge"
        }
    }
}

Example OR

{
    "collection": "grafana.promql.templates",
    "query": "count over time",
    "filter": {
        "$or": [
            {
                "metric_type": {
                    "$eq": "gauge"
                }
            },
            {
                "metric_type": {
                    "$eq": "histogram"
                }
            }
        ]
    }
}

[sd2k]

A few comments inline, haven't tested this but it seems good otherwise.

[sd2k]

LGTM