feat: Group key and encryption encoding into one function

[danieljbruce]

This pull request is dependent on https://github.com/googleapis/nodejs-bigtable/pull/1093. This PR is the 4th of a series of PRs for refactoring the conversion of cluster input parameters key and encryption into one function so that we don't have repeated code. Still more needs to be done like we should do the same for the storage parameter and we should also think about doing these conversions for cluster updates as well though breaking changes are involved with such tasks so those changes should be considered carefully.