[DO NOT MERGE] GoodMem chat plugin for ADK

[forrestbao]

PLEASE DO NOT MERGE THIS PR. This PR is for GoodMem team to clearly see what files are changed/added. Later, we will create a formal PR and submitted to google's adk-python-community repo.

GOODMEM TEAM: Please make changes in the plugin branch.

Add GoodmemChatPlugin for persistent conversation memory using Goodmem.ai
service. The plugin automatically logs user messages and LLM responses,
retrieves relevant historical context, and augments prompts with memory.

Features:

• Auto-logging of text messages and file attachments to Goodmem spaces
• Context retrieval using semantic search with configurable top-k
• Per-user memory isolation with automatic space management
• Comprehensive error handling and debug mode
• Support for binary attachments (PDFs, images, etc.)

File structure:

.
├── src/google/adk_community/
│   ├── __init__.py                      (modified: added plugins import)
│   └── plugins/
│       ├── __init__.py                   (modified: updated imports to use goodmem submodule)
│       └── goodmem/
│           ├── __init__.py               (new: module exports)
│           ├── goodmem_client.py         (new: 274 lines, HTTP client for Goodmem API)
│           └── goodmem.py                (new: 593 lines, plugin implementation)
│
├── tests/unittests/
│   └── plugins/
│       ├── __init__.py                   (new: test module)
│       └── test_goodmem.py               (new: 31 unit tests, 712 lines)
│
└── contributing/samples/goodmem/
    ├── README.md                         (new: overview of Goodmem integrations)
    ├── PLUGIN.md                         (new: detailed plugin documentation)
    └── goodmem_plugin_demo/
        └── agent.py                      (new: sample agent with plugin)

Test coverage: 31/31 tests passing (100%)

• 8 tests for GoodmemClient (API operations)
• 23 tests for GoodmemChatPlugin (lifecycle, callbacks, error handling)

Related: Follows patterns from OpenMemory (PR google#36) and Redis Sessions (PR google#32)

[forrestbao]

This one is ready to be submit to Google.
I extensively talked with AI to find other places to fix/improve. For some, I left untouched because either we don't have a good solution yet or it is unlikely to happen. They are listed as "known limitation" in Readme:

## Known limitations
1. Backend enforces max 10,000 characters on query message. But the plugin doesn't validate this and will silently pass the rejection from Goodmem. 
2. Backend has a binary file size limit of 1GB. But the plugin doesn't validate this and will silently pass the rejection from Goodmem. 
3. Backend enforces max 50 metadata keys. But the plugin doesn't validate this and will silently pass the rejection from Goodmem. 
4. No rate limit handling. Backend returns HTTP 429 with Retry-After header. The plugin will silently ignore it. 
5. The plugin does not check whether ingesting the memory is finished -- ingestion takes a while. It will silently ignore if it eventually fails. 

[forrestbao]

Close this branch as we are using tools_and_plugin to submit to Google.