Add missing RDS paginators

[brandond]

Looks like someone added a paginator for DescribeDBClusters but didn't add paginators for any of the other Cluster actions.

[brandond]

@stealthycoin @JordonPhillips seems like #1462 is a better long-term approach, as opposed to asking folks to manually maintain the paginators separate from the service definitions.

[codecov-io]

Codecov Report

Merging #1548 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1548   +/-   ##
========================================
  Coverage    91.99%   91.99%           
========================================
  Files           51       51           
  Lines         9335     9335           
========================================
  Hits          8588     8588           
  Misses         747      747

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[JordonPhillips]

Auto-generating isn't foolproof - if the response contains anything but a list and the pagination token then it isn't safe to guess the result key. I know a while ago we actually did go through and auto-gen a bunch of them, but we haven't done so since. It would probably be worth doing that. In the meantime, this pr looks good so I'll go ahead and merge it.