Missing get Actor default build method

[drobnikj]

The API allows getting the default build of the Actor using
GET /v2/acts/${actorId}/builds/default
This method is not supported by the client.

There are two options how to do it in the client:

• ApifyClient.actor('my-actor').getBuild('default') // or .getBuild('0.1.2') or .getBuild('latest')
• ApifyClient.actor('my-actor').defaultBuild()

Since we have .lastRun(), for consistency, it would make sense to have .defaultBuild().

We should do this in the Python client too.