[CAP] Added a factory for runtime

[rajan-chari]

Why are these changes needed?

The default runtime for CAP uses ZeroMQ. This PR introduces a runtime factory which can be used for registration and usage of other runtimes.

Related issue number

Checks

• [x] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.

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GitGuardian id	GitGuardian status	Secret	Commit	Filename
10404695	Triggered	Generic High Entropy Secret	3357b3ea1bd418c2e3e97fd99e7e1fe3e1a4d8d1	test/oai/test_utils.py	View secret

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