feat: add built-in duration type

[RutZap]

The Duration type is a first-class Keel model type and will capture an interval of time. For example:

model Employee {
	fields {
	  name Text
		startDate Date
		periodOfService Duration
	}
}

The duration field will accept ISO 8601 durations, e.g. P1Y10M4DT10H2M1S. These data types will be stored in the DB as Postgres' Interval types.

When generating the JSON schema for the Keel app's API, these fields will be of type: string and format duration.

Filter support

When a Duration field is defined as a filterable field for a list action, the input message will support the following fields/operations:

{
"myDuration": {
      "equals": "duration",
      "notEquals": "duration",
      "lessThan": "duration",
      "lessThanOrEquals": "duration",
      "greaterThan": "duration",
      "greaterThanOrEquals": "duration",
 }

The resolution of these operations will be deferred to the DB engine.

---

ModelAPI & Functions Runtime support

We're introducing a new class in the keel functions runtime sdk: Duration and built-in support in the modelAPI.

Considering the following keel model schema:

model MyDuration {
    fields {
        dur Duration?
    }
}

We are able to interact with the dur field through the sdk.Duration class:

import { WriteCustomFunction, models, Duration } from "@teamkeel/sdk";

export default WriteCustomFunction(async (ctx, inputs) => {
  const mod = await models.myDuration.create({ dur: Duration.fromISOString("PT1H") });
  const dbModel = await models.myDuration.findOne({ id: mod.id})
  return {
    isoDuration: dbModel.dur.toISOString(),
  }
});