[SDK] add tool decorator helper for custom client tools

[yanxi0830]

What does this PR do?

• Add decorator for on callables for defining client side custom tools
• Addresses: Support tool definition via decoration llama-stack#948

Test Plan

Usage:

@client_tool
def add(x: int, y: int) -> int:
    '''Add 2 integer numbers
    
            :param x: integer 1
            :param y: integer 2
            :returns: sum of x + y
    '''
    return x + y

add will be a ClientTool that can be passed

• Working example in: feat: ReACT agent example llama-stack-apps#166

Sources

Please link relevant resources if necessary.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.