Set default `frame-ancestors` on default Content-Security-Policy

We should consider setting a default `frame-ancestors` directive for the Content Security Policy. The [`frame-ancestors`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors) directive is the new iteration of the X-Frame-Options header, and as such setting a directive in both spots might be prudent.

https://github.com/github/secure_headers/blob/b134eef07d3741b4bd0769b863961b41af5df57d/lib/secure_headers/headers/content_security_policy_config.rb#L97

Since our default XFO policy is `sameorigin`, if we decide to take upon this task, we should set the default `frame-ancestors` value to be `self`.

Some counterpoints: setting both the `X-Frame-Options` and the `frame-ancestors` directive will cause the XFO header to be overriden by the frame-ancestors directive. This means that if a user is trying to change some framing functionality, and only changes the XFO header, they might be confused as to why the functionality didn't actually change.

Start with lib/secure_headers/headers/content_security_policy_config.rb around the linked default policy. Review how the existing sameorigin X-Frame-Options default is configured and consider the stated interaction with frame-ancestors. Done means resolving whether the default should include frame-ancestors self and implementing the decision without leaving the two framing controls misleadingly inconsistent.

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