Add Permissions-Policy directive to control FLoC

[doktorbro]

google’s floc is coming. it’s time to react.

• eff said: google’s floc is a terrible idea
• brave said: why brave disables floc
• wordpress said: treat floc like a security concern

the apache config to opt out the floc is:

<IfModule mod_headers.c>
  Header always set Permissions-Policy "interest-cohort=()"
</IfModule>

[LeoColomb]

Thanks a lot for opening this issue @doktorbro!

While I'm personally in favor of blocking FLoC, I'm not sure about its addition into H5BP.

• This is quite political for now, and it's probably better to wait a bit.
• Especially regarding current web ecosystem reaction, it might be useless in few months (FLoC is "only" a experiement).
• This addition won't be enabled by default, reducing its value in short-term and its coverage.
• And finally there is already a issue regarding Permissions-Policy header: Permissions-Policy header #179.

[doktorbro]

@LeoColomb if it’s better to wait then i will.

[LeoColomb]

I'm closing this one in favor of #179