[ADK] Add pluggable memory middleware based on Retriever/Indexer/Embedding

## Summary

I would like to propose a pluggable memory middleware for Eino ADK agents.

Eino already provides core component abstractions such as `Retriever`, `Indexer`, and `Embedding`, and ADK already provides middleware hooks around agent/model execution. However, there is currently no standard ADK-level memory middleware that connects these pieces into a reusable agent memory workflow.

## Motivation

When building long-running or general-purpose agents on top of Eino, application developers often need:

- Retrieve relevant long-term memory before model invocation
- Inject retrieved memory into the model context in a controlled format
- Write important facts, user preferences, or task summaries back to memory
- Support memory namespaces such as user/session/agent/project
- Keep memory provider implementations outside of Eino core

Without a standard middleware abstraction, each application has to implement its own memory injection, memory write policy, and storage integration.

## Proposed Design

Add an ADK memory middleware abstraction that supports:

- `BeforeModelRewriteState`: retrieve and inject memory into model input
- `AfterModelRewriteState` or event-based hooks: extract/write memory after model or agent output
- Configurable memory namespace resolver
- Configurable memory formatting
- Configurable memory write policy
- Integration with existing `Retriever`, `Indexer`, and `Embedding` interfaces
- No dependency on a specific vector database in Eino core

A rough API could look like:

```go
type MemoryStore interface {
Search(ctx context.Context, query string, opts ...MemoryOption) ([]*schema.Document, error)
Save(ctx context.Context, docs []*schema.Document, opts ...MemoryOption) error
}

type MemoryMiddlewareConfig struct {
Store MemoryStore
NamespaceResolver func(ctx context.Context, input *adk.AgentInput) string
QueryBuilder func(ctx context.Context, messages []*schema.Message) string
MemoryFormatter func(ctx context.Context, docs []*schema.Document) []*schema.Message
WritePolicy func(ctx context.Context, event *adk.AgentEvent) ([]*schema.Document, bool, error)
}
```

## Expected Benefits

- Makes Eino ADK easier to use for long-running agents
- Encourages a common memory pattern across Eino applications
- Keeps vector DB implementations in `eino-ext` while providing the reusable ADK abstraction in Eino core
- Helps framework users build agent systems with durable memory without reinventing middleware logic

## Related Context

This is not a request to add a specific vector database implementation to Eino core. The request is for a general ADK middleware abstraction that can work with existing Eino component interfaces and external memory backends.