Support for object deployment payloads

[ewiegs4]

Description

Deployments contain a payload field that is described as:

JSON payload with extra information about the deployment.

This library's GHDeployment.getPayload() method always tries to cast the deserialized payload object into a String, which fails for any payload that is a JSON object (deserialized as a Map).

While a particularly elegant solution might be to allow specifying a custom Java type to deserialize the payload into, currently object payloads are completely inaccessible so I am proposing a minimal solution of allowing access to the payload as the raw Object or as a Map<String, Object>.

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[bitwiseman]

Not a super elegant solution, but perfectly reasonable.

Sufficient tests.

Great! Thanks!