Add a field-based constraint for timestamps

[popematt]

Currently, you can create a constraint on the n lowest precision fields by restricting a timestamp with a range (ie. using valid_values), and you can restrict the n highest precision fields to be 0 by placing a restriction on the timestamp precision. However, you cannot constrain an arbitrary field within a timestamp without placing a constraint on any of it's "neighboring" fields (eg. you can't constrain days without also constraining either months or hours) nor can you constrain a high-precision field to be something other than 0. It would be useful to be able to define constraints on specific fields within a timestamp.

Example use cases:

1. All timestamps in your data model must have seconds precision (by chosen convention, or perhaps because of some framework in use), but you want to restrict a certain value to have minutes and seconds of zero.

type::{
  name: my_app_timestamp,
  timestamp_precision: seconds,
}

type::{
  name: start_of_hour_timestamp,
  type: my_app_timestamp,
  timestamp_fields: {
    minute: 0,
    seconds: 0
  }
}

2. Timestamps represent buckets that have a specific granularity

type::{
  name: five_minute_bucket,
  timestamp_fields: {
    minute: [0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55],
    second: 0,
  }
}