Add PDQHash and PDQHashF to Public Image Hashers

[mjs2600]

PDQHash and PDQHashF were unintentionally left off of the list of public hashers. This adds them to the list.

[davidrs]

README may need an update since it says a different path pdq

[faustomorales]

FWIW, leaving PDQ out was intentional. The reason was that pdqhash-python doesn't always build reliably and I didn't want someone to be blocked from using perception just because they couldn't get pdqhash-python to work. I believe that the change in this PR, as it stands, would make it impossible to use perception without pdqhash-python being installed successfully. But maybe that's what's best! I just wanted to make sure that outcome was considered.

[mjs2600]

Thanks for the context @faustomorales! I'll rework this and move pdq into an extra in case someone doesn't want or need it.