Add support for Speed Brain

[cf-ypark]

Description

Add support for Speed Brain feature

Has your change been tested?

Unit tests have been added that align with the documented API spec.

Screenshots (if appropriate):

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas
  and relies on stable APIs.

[jeehoon-cf]

Is this OK to be placed in root, not under /internal ?

[cf-ypark]

is there a reason why it should fall under /internal?

[jeehoon-cf]

I was looking at different repo.

[cf-ypark]

not sure why changelog-check is failing @jacobbednarz any help?

[jacobbednarz]

i don't think we need this PR as it would already be supported using https://github.com/cloudflare/cloudflare-go/blob/master/zone.go#L928-L985. terraform we should just be able to add the key to the array have this Just Work™

[cf-ypark]

got it, @jacobbednarz I'll close this one then. thanks!