Add param support for payload configurations

[joshbrucker]

What is the purpose of this pull request?

• Bug fix
• Documentation update
• Code cleanup
• New feature
• Other (please explain):

Addresses #
N/A

What changes did you make? (Give a brief overview)
For every GitHub webhook, there are payload settings that can be configured. These are the payload destination address, the payload serialization format, and a payload secret. This PR adds these three fields as configurable values in the put step of the Concourse resource. All fields are optional, and if they are not explicitly passed in, the behavior remains the same as it currently is (pulled from env vars set by Concourse).

Also, in the config parameter passed into the GitHub API call, the key content-type has been changed to the correct key content_type, as demonstrated in the GitHub API docs.

Is there anything specific you would like reviewers to focus on?
N/A

[joshbrucker]

@billimek @GavinFigueroa The above feature has been implemented and tested - it's now ready for review.

[billimek]

LGTM

[billimek]

Thanks for making this contribution!