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 dual-read/write migration paths can affect customer data routing when adopted; sync builder refactor is low-risk but touches released migration APIs.

Overview
Adds an experimental async migration API (AsyncMigrator, AsyncMigratorBuilder, AsyncMigrationConfig, AsyncMigratorFn) that mirrors the sync migrator: flag-driven stages, old/new coroutine hooks, dual-read consistency checks, and track_migration_op telemetry. Reads can run in parallel via asyncio.gather (vs threads on sync).

Sync refactor: fluent builder options (read_execution_order, track_latency, track_errors) move to _MigratorBuilderBase in types.py; MigratorBuilder subclasses it with unchanged behavior. New symbols are eagerly exported from ldclient.migrations (stdlib-only import path).

Broad test_async_migrator.py coverage aligns with existing sync migrator tests (stages, tracking, execution order).

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[jsonbailey]

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