Add runLoop function

[aryan-25]

Motivation:

As part of the implementation of the new v2 API (#339), the LambdaRuntimeClientProtocol was introduced in #344.

Building upon it, this PR introduces a static runLoop function that implements the runtime loop (fetch the next event -> handle the event -> report the result -> repeat).

This function is generic over an object conforming to LambdaRuntimeClientProtocol to also allow for mock runtime clients to be used in tests.

Modifications:

• Add the StreamingLambdaHandler protocol as detailed in the v2 API proposal (v2 API Proposal Document #339).
• Add the static runLoop function.

Result:

Runtime loop implemented. The library can also be tested more easily, e.g. through a mock runtime client object passed to runLoop.

[fabianfett]

Great change! Thanks for pushing through!