Adding util to help developers add message attachment

[vladimirivic]

Summary:
Adding MessageAttachment.base64(path) so that sending attachment will look like this:

from llama_stack_client.lib.inference.utils import MessageAttachment

response = client.inference.chat_completion(
    model_id="meta-llama/llama3.2-11b-vision-instruct",
    messages=[
        {
            "role": "user",
            "content": {
                "type": "image",
                "image": {
                    "data": MessageAttachment.base64("images/tennis-game.png")
                }
            }
        },
        {
            "role": "user",
            "content": "What's in this image?",
        }
    ]
)

Test Plan:

pip install .

# start a new notebook and run

from llama_stack_client import LlamaStackClient
from llama_stack_client.lib.inference.utils import MessageAttachment

client = LlamaStackClient(
    base_url='localhost:8321'
)

response = client.inference.chat_completion(
    model_id="meta-llama/llama3.2-11b-vision-instruct",
    messages=[
        {
            "role": "user",
            "content": {
                "type": "image",
                "image": {
                    "data": MessageAttachment.base64("images/tennis-game.png")
                }
            }
        },
        {
            "role": "user",
            "content": "What's in this image?",
        }
    ]
)

print(response)