Feature request: a DateTime scalar

[sagikazarmark]

Dagger exposes the git committer date as a String! ("Git committer date, in
RFC3339 format"). I want to use it as SOURCE_DATE_EPOCH for reproducible
builds, which means parsing that string and turning it into a Unix timestamp.
Today there is no way to do that from Dang.

Example:

build(src: Directory!, ref: GitRef!): Container! {
  let epoch = DateTime(ref.commitDate).unix
  dag.container.from("alpine")
    .withDirectory("/src", src)
    .withEnvVariable("SOURCE_DATE_EPOCH", toString(epoch))
}

Proposed API

Construction

let d = DateTime(ref.commitDate)           # constructor, for non-literal Strings
let d: DateTime! = "2024-03-01T12:00:00Z"  # literals auto-coerce into a DateTime slot
let d = ref.commitDate :: DateTime!        # explicit cast

• Accepts RFC 3339 only (Go time.RFC3339; fractional seconds allowed).
  This is exactly what git's %cI and Dagger's field produce.
• Invalid input raises a recoverable error at the slot, like Path/Regexp:
  DateTime: invalid RFC 3339 timestamp "yesterday" (works with rescue).
• Canonical form: the parsed value re-rendered with time.RFC3339Nano,
  offset preserved. For git dates this is a no-op (+00:00 → Z is the
  only visible normalization). Same normalize-on-construction idea as Path.
• JSON / YAML / TOML.decode into a DateTime!-typed field goes through
  the same hook automatically (existing codec scalar path).

Statics (DateTime.x)

Member	Doc
DateTime.now: DateTime!	the current instant, in UTC (host-TZ independent, so the string form is deterministic across machines)
DateTime.fromUnix(seconds: Int!): DateTime!	the instant seconds after the Unix epoch, in UTC

Methods (d.x)

Member	Doc
unix: Int!	seconds since the Unix epoch
isBefore(other: DateTime!): Boolean!	instant comparison
isAfter(other: DateTime!): Boolean!	instant comparison
string: String!	the canonical RFC 3339 text (mirrors Path.string)

Exit is free under the existing scalar rules: a DateTime! degrades to
String! at any value-handoff boundary, and toString / interpolation yield
the RFC 3339 text, so it can be passed straight back into any Dagger String!
argument.