Add MaxItems attribute to Schema

[vancluever]

Hey guys,

This PR adds a new field for use in a resource Schema: MaxItems.

From the commit log:

 * MaxItems defines a maximum amount of items that can exist within a
   TypeSet or TypeList. Specific use cases would be if a TypeSet is being
   used to wrap a complex structure, however more than one instance would
   cause instability.

Of course, this can be used to limit the size of plain old lists, or more general-use sets, just in case a request size does need to be limited for some reason.

I created this to support my new aws_cloudfront_distribution resource, which has several complex structures in it that will behave badly if you specify more than one instance of a structure that is only expected to have a single element. See comments on #3330. I have tested this with the acceptance tests on my CloudFront branch, and also have included unit tests for the functionality itself.

The plan being once this is accepted I will add this to the aws_cloudfront_distribution resource appropriately.

[phinze]

Since this error message will end up being user facing, let's make the language verbose so the user understands exactly what they need to fix.

Something like (w/ an in-context example):

aws_instance.foo: root_block_device: attribute supports 1 item maximum, config has 2 declared

Maybe we can think of some clearer language, but just something to help users correct their config when they see the error is what I'm getting at.

[phinze]

This is looking good! Let's get the error message figured out and then this will be good to go. 👌

[vancluever]

attribute supports 1 item maximum, config has 2 declared

Sounds good enough to me. :) Updated and squashed!

[apparentlymart]

Looking good. Thanks!

[vancluever]

Thanks @apparentlymart! Now I can add this to #5221.

[ghost]

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