Extension mechanism

[symphony-mariacristina]

Description

The Extension API is available through the module symphony.bdk.core.extension. Extensions are registered programmatically via the ExtensionService. A Service Provider extension is a specific type of extension loaded on demand when calling the ExtensionService#service(MyExtensionType) method.

To make your extension Service Provider, your extension definition must implement the method get_service(self).

# Usage example.
async def run():
    config = BdkConfigLoader.load_from_symphony_dir("config.yaml")
    async with SymphonyBdk(config) as bdk:
        extension_service = bdk.extensions()
        extension_service.register(MyBdkExtensionService)
        service = extension_service.service(MyBdkExtensionService)
        service.say_hello("Symphony")
        
asyncio.run(run())

BDK Aware Extensions

The BDK Extension Model allows extensions to access to some core objects such as the configuration or the api clients.
Developers that wish to use these objects are free to implement a set of abstract base classes all suffixed with the Aware keyword.
List of Aware classes:

• BdkConfigAware- allows extensions to read the BdkConfig:
• BdkApiClientFactoryAware - can be used by extensions that need to use the ApiClientFactory
• BdkAuthenticationAware- can be used by extensions that need to rely on the service account authentication session (AuthSession)

Symphony Groups Extension

As part of this PR we also leveraged this new Extension Mechanism to implement a new custom extension to support the Symphony Groups APIs.

[symphony-youri]

I was going to say we should name it create group to be consistent with other services but then in the Java BDK it is named like that, I missed it during the Java BDK review :(

[symphony-youri]

Mmm I missed that in the Java BDK we were doing that (guessing the tenant id from the user id) (which the profile manager API should do for us already...)
So I should probably change the WDK implem too (although we only do it partially for this method and not for the creation)

[symphony-elias]

@symphony-mariacristina should we document how to leverage the retry mechanism in extensions? I would say so because it differs much from the Java BDK

[symphony-elias]

LGTM

[symphony-elias]

Implements #251