Add Service Credential Bindings of type Key

[ddraganovv]

Is there a related GitHub Issue?

https://github.com/cloudfoundry/korifi/issues/2320

What is this change about?

With this PR we are introducing a new type of service binding - 'key'.

Does this PR introduce a breaking change?

No

Acceptance Steps

Tag your pair, your PM, and/or team

[linux-foundation-easycla[bot]]

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