missing validation of attribute names containing upper case characters

Describe the Bug

register_type accepts attribute names containing upper case characters. Puppet will happily downcase those for internal processing, without RSAPI noticing. this in turn leads to random-looking failures later in processing.

Expected Behavior

RSAPI register_type and register_transport refuse to accept attribute/connection_info keys that do not conform to all-lower-case snake casing rules (all lowercase, numbers, underscores, starts with letter).

Steps to Reproduce

Steps to reproduce the behavior:

1. Create a new type called some with a mandatory attribute called someThing
2. Run puppet apply -e 'some { foo: }

Will fail with:

david@davids:~/tmp/foo$ pdk bundle exec puppet apply -e 'some { test: }' --verbose --trace --modulepath spec/fixtures/modules
pdk (INFO): Using Ruby 2.5.7
pdk (INFO): Using Puppet 6.10.1
Notice: Compiled catalog for localhost in environment production in 0.11 seconds
Info: Applying configuration version '1573033174'
Error: /Stage[main]/Some[test]: Could not evaluate: The following mandatory attributes were not provided:
    *  SomeThing
/opt/puppetlabs/pdk/private/puppet/ruby/2.5.0/gems/puppet-resource_api-1.8.7/lib/puppet/resource_api.rb:342:in `raise_missing_attrs'
/opt/puppetlabs/pdk/private/puppet/ruby/2.5.0/gems/puppet-resource_api-1.8.7/lib/puppet/resource_api.rb:289:in `retrieve'
/opt/puppetlabs/pdk/share/cache/ruby/2.5.0/gems/puppet-6.10.1/lib/puppet/type.rb:1133:in `retrieve_resource'
[...]

Additional Context

This problem was originally reported and diagnosed by Brian Dinga on the community slack.

Search for the register_type and register_transport entry points and reproduce the reported mixed-case attribute failure. Verify the existing registration behavior for attribute and connection_info keys, then confirm that invalid names are refused according to the stated lowercase snake-case rules. No specific files or tests are named in the issue.