feat(Async): Add IAsyncConfigurationReader + ParseAsync overload

[dimension-zero]

feat(Async): Add IAsyncConfigurationReader + ParseAsync overload

• ConfigReaders.fs:
  • New IAsyncConfigurationReader interface mirroring
    IConfigurationReader with GetValueAsync : string -> Task.
  • ConfigurationReader.AsAsync(reader) wraps any sync reader as async
    via Task.FromResult — useful when callers want to pass existing
    readers through the async parse path.
  • ConfigurationReader.FromAsyncFunction(asyncFn, ?name) builds a
    reader from an F# Async.
• ArgumentParser.fs new ParseAsync overload: fetches every top-level
  AppSettings key the schema references via the async reader, awaits
  each, then runs the regular sync Parse against a Dictionary-backed
  snapshot. One round-trip per declared key; below ParseAsync, the
  parser stays purely synchronous so existing tests and behaviour are
  unaffected.

Hosts that previously had to block on an async config source can now
use ParseAsync directly:
do! parser.ParseAsync(configurationReader = remoteAsync) |> Async.AwaitTask

---

test(ParseAsync): Coverage for IAsyncConfigurationReader + ParseAsync

5 new tests:

• ConfigurationReader.AsAsync wraps a sync reader; the wrapped Task
  resolves to the same value and preserves Name.
• ParseAsync via AsAsync(sync reader) yields the same parse results as
  the original sync Parse against that reader.
• ParseAsync pre-fetches each schema-declared AppSettings key exactly
  once (Interlocked-counted via a custom IAsyncConfigurationReader),
  guarding the implementation's contract.
• ConfigurationReader.FromAsyncFunction adapts an F# Async
  to IAsyncConfigurationReader and parses round-trip.
• ParseAsync forwards ignoreUnrecognized to the underlying sync Parse
  (CLI tokens not in the schema land in UnrecognizedCliParams).

Net suite size on this branch: 112 -> 117.

---

[nojaf]

@bartelink do you have some bandwidth to review these PRs?
I believe you have dealt with the codebase in the past.

[bartelink]

lol, did T-Gro mention I'm atting him too much and need to take a break :D

The PRs thankfully look pretty reviewable but ngl I was hoping they'd magically get merged by 'someone'!

But yes, I do have space atm so I'll chip away at them