refactor coder credentials to load classes dynamically

[johrstrom]

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What does this PR do?

This changes the way the coder adapter's credentials are loaded so that they can be dynamically loaded at runtime without using a predefined case statement to load them. That is, users/centers can define these classes then just utilize them without contribution to this library.

Related issue

Closes #___ (if applicable)

Testing

• Tests included
• No tests needed — reason: ___

Checklist

• Follows project code style and conventions
• Documentation provided (if new feature, adapter or behavior change)
• This is a large feature and was discussed in an issue first (if applicable)

Anything else?

@andrejcermak & @Satyam-Captain FYI

[johrstrom]

Note that typical ruby classes correspond to the file name with _ being transformed into capitalizations.

I.e., this file really should have been open_stack_credentials with the classname OpenStackCredentials.

So I had to change the classname so that the configuration of openstack continues to work (otherwise it would have to be changed to open_stack.