Add missing paginator definitions for CloudFormation

[slai]

This adds the missing pagination definitions for the CloudFormation APIs, as described in #1462.

I've tested DescribeChangeSet and ListChangeSets and both work as expected after this change,

client = boto3.client('cloudformation', region_name='eu-west-1')

paginator = client.get_paginator('list_change_sets')
page_iterator = paginator.paginate(StackName='MyStack')

for page in page_iterator:
    print(page)

[codecov-io]

Codecov Report

Merging #1594 into develop will increase coverage by 0.23%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1594      +/-   ##
===========================================
+ Coverage       92%   92.24%   +0.23%     
===========================================
  Files           50       52       +2     
  Lines         9110     9592     +482     
===========================================
+ Hits          8382     8848     +466     
- Misses         728      744      +16

Impacted Files	Coverage Δ
botocore/session.py	98.07% <0%> (-0.99%)	⬇️
botocore/exceptions.py	99.39% <0%> (-0.61%)	⬇️
botocore/utils.py	97.62% <0%> (-0.3%)	⬇️
botocore/handlers.py	97.04% <0%> (-0.1%)	⬇️
botocore/config.py	100% <0%> (ø)	⬆️
botocore/serialize.py	100% <0%> (ø)	⬆️
botocore/args.py	100% <0%> (ø)	⬆️
botocore/configprovider.py	93.45% <0%> (ø)
botocore/monitoring.py	99.51% <0%> (ø)
botocore/client.py	99.75% <0%> (ø)	⬆️
... and 6 more

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[joguSD]

All of these look good except for DescribeChangeSet. That operation has quite a few fields in the output in addition to the Changes result_key. I'm pretty sure they could all be added as non_aggregate_keys, but a couple of them are lists which is making me a little hesitant right now. I'd need to first confirm that the auxiliary lists are also not paginated, which I don't know off hand.

We would need to ensure that all output keys are accounted for, otherwise they'll be dropped from build_full_results output, which in turn effects the CLI in a backwards incompatible way.

[joguSD]

Merged in 3b2a60b