API for raw JSONPATH queries

[roji]

#28616 tracks translating LINQ operators inside JSON documents to JSONPATH. But just like we have raw SQL query APIs, we should also have a raw JSONPATH API, which allows the user to directly provide a JSONPATH string query and get results out. This would act as an escape hatch API for when we don't support a LINQ translation.

Ideal LINQ construct (would be possible as part of #28616):

ctx.Entity.Where(e => e.JsonThing.Track.Segments.Any(s => s.HR > 130))

Escape hatch with raw JSONPATH:

ctx.Entity.Where(e => EF.Functions.Jsonpath<Segment>(e.JsonThing, "$.track.segments[*].HR ? (@ > 130)"))

We may need different functions, one for returning an enumerable and another for returning a single thing (e.g. JSON_VALUE vs. JSON_QUERY in SQL Server).

[ajcvickers]

Note from triage: @ajcvickers to test and document the wokaround.

[roji]

Given that JSONPATH support is very limited across databases (see #28616 (comment)), this probably doesn't make sense as a relational feature. PostgreSQL can implement support for this as a provider-specific translation.