Allow binding parameters to override credentials

[f0rmiga]

Following up on #99 (comment), it was very hidden that it would be possible to override the credentials while binding. We need a proper mechanism for this and have clear documentation on how it works.

An option would be to allow a key overrides to be passed to the binding request:

{ "overrides": { "mongodbDatabase": "foo" } }

This is useful if somehow the user created extra databases for other apps.

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[k8s-ci-robot]

@fejta-bot: Closing this issue.

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