add support for Custom Hostnames without SSL

[nickysemenza]

This change updates the client to match the fact that the ssl is an optional (yet almost always used) attribute on the custom hostnames API. Breaking change since an existing struct field was modified.

One caveat/part I haven't implemented here is that to turn off SSL for a hostname, you have to PATCH with a payload of {"hostname: "x", "ssl":"null"}, quotes important, simply relying on omitempty ({"hostname: "x"}) is not enough.

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.

[mihirjham]

Looks good to me.
@jacobbednarz will you help us with reviews and merging?

[jacobbednarz]

as we're not 1.0 yet it think this is fine as a minor release with some notice in the release notes to show that we've changed the type to a pointer. the terraform work will be the most annoying part of this change.