feat: Add async migrations

[jsonbailey]

Adds the async migration surface for the async Python SDK — AsyncMigrator, AsyncMigratorBuilder, AsyncMigrationConfig, and the AsyncMigratorFn type — in ldclient/migrations/async_migrator.py, eagerly exported from ldclient/migrations/__init__.py (import-cheap: asyncio stdlib only, no aiohttp).

Also carries the migration config/builder dedup shared by the sync and async surfaces: types.py gains _MigrationConfigBase[_MigratorFnT] (Generic) and _MigratorBuilderBase (fluent-setter mixin), and migrator.py's MigratorBuilder is re-based onto that mixin. These are behavior-preserving refactors of already-released sync code.

The public async classes carry experimental .. caution:: blocks.

Self-contained: depends only on already-merged foundation work. The AsyncLDClient/Context references are TYPE_CHECKING-only (annotations kept lazy via from __future__ import annotations); no runtime import of any not-yet-merged async module.

No CHANGELOG or version changes (handled at release).

Tracked internally: SDK-2767

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Note

Medium Risk
New experimental API affects migration read/write orchestration and dual-write behavior; sync builder refactor is intended behavior-preserving but touches released migration configuration code.

Overview
Adds an experimental async migration API (AsyncMigrator, AsyncMigratorBuilder, AsyncMigrationConfig, AsyncMigratorFn) that mirrors the sync migrator: flag-backed stage selection via await migration_variation, coroutine-based old/new read/write handlers, dual-origin reads with asyncio.gather for parallel execution, and synchronous track_migration_op after each operation. Public types are exported from ldclient.migrations with a cheap import path (stdlib asyncio only).

Refactors shared sync/async plumbing in types.py via _MigrationConfigBase and _MigratorBuilderBase; MigratorBuilder now inherits the mixin (fluent read_execution_order / track_latency / track_errors unchanged, attribute names normalized to _read_execution_order etc.).

Adds broad test_async_migrator.py coverage aligned with sync migrator tests (stages, tracking, consistency, exceptions, execution order).

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[jsonbailey]

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