I modified the proxy host cache we already had and were using for
websocket csp headers to also include the wildcard app host, then used
those for frame-src policies.
I did not add frame-ancestors, since if I understand correctly, those
would go on the app, and this middleware does not come into play there.
Maybe we will want to add it on workspace apps like we do with cors, if
we find apps are setting it to `none` or something.
Closes https://github.com/coder/internal/issues/684
Allows adding custom static CSP directives to Coder. Niche use case but
makes this easier then creating a reverse proxy that has to replace the
header. We want to preserve our directives, so having an append option
is preferred to a "replace" option via a reverse proxy.
Closes https://github.com/coder/coder/issues/15118
