Moving all TOS logic into a single file

[dparker1005]

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Changes proposed in this Pull Request:

Currently, the logic for TOS fields is spread across numerous PMPro files when the TOS field can be implemented using existing PMPro hooks.

Moving all of the TOS logic into a single file has a number of benefits:

1. Simplifies all of the code that included TOS logic (specifically checkout) and will allow for more flexibility when updating those files in the future.
2. Allows unhooking the TOS field from the checkout page and re-hooking a custom UI (such as embedding the page contents into the checkout form)
3. Makes it easier to extend the TOS field in the future by having all the code that would need to be modified in a single place
4. Provides a template for creating a second TOS field using custom code.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

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