[feature] proof of work scraper deterrence (#4043)

This adds a proof-of-work based scraper deterrence to GoToSocial's middleware stack on profile and status web pages. Heavily inspired by https://github.com/TecharoHQ/anubis, but massively stripped back for our own usecase.

Todo:
- ~~add configuration option so this is disabled by default~~
- ~~fix whatever weirdness is preventing this working with CSP (even in debug)~~
- ~~use our standard templating mechanism going through apiutil helper func~~
- ~~probably some absurdly small performance improvements to be made in pooling re-used hex encode / hash encode buffers~~ the web endpoints aren't as hot a path as API / ActivityPub, will leave as-is for now as it is already very minimal and well optimized
- ~~verify the cryptographic assumptions re: using a portion of token as challenge data~~ this isn't a serious application of cryptography, if it turns out to be a problem we'll fix it, but it definitely should not be easily possible to guess a SHA256 hash from the first 1/4 of it even if mathematically it might make it a bit easier
- ~~theme / make look nice??~~
- ~~add a spinner~~
- ~~add entry in example configuration~~
- ~~add documentation~~

Verification page originally based on https://github.com/LucienV1/powtect

Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4043
Reviewed-by: tobi <tsmethurst@noreply.codeberg.org>
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

example/config.yaml

@@ -1261,3 +1261,22 @@ advanced-csp-extra-uris: []
 # Options: ["block", "allow", ""]
 # Default: ""
 advanced-header-filter-mode: ""
+
+# Bool. Enables a proof-of-work based deterrence against scrapers
+# on profile and status web pages. This will generate a unique but
+# deterministic challenge for each HTTP client to complete before
+# accessing the above mentioned endpoints, on success being given
+# a cookie that permits challenge-less access within a 1hr window.
+#
+# The outcome of this is that it should make scraping of these
+# endpoints economically unfeasible, while having a negligible
+# performance impact on your own instance.
+#
+# The downside is that it requires javascript to be enabled.
+#
+# For more details please check the documentation at:
+# https://docs.gotosocial.org/en/latest/admin/scraper_deterrence
+#
+# Options: [true, false]
+# Default: true
+advanced-scraper-deterrence: false