FEATURE: Persist password hashing algorithm/params in database (#20980)

Previously, Discourse's password hashing was hard-coded to a specific algorithm and parameters. Any changes to the algorithm or parameters would essentially invalidate all existing user passwords.

This commit introduces a new `password_algorithm` column on the `users` table. This persists the algorithm/parameters which were use to generate the hash for a given user. All existing rows in the users table are assumed to be using Discourse's current algorithm/parameters. With this data stored per-user in the database, we'll be able to keep existing passwords working while adjusting the algorithm/parameters for newly hashed passwords.

Passwords which were hashed with an old algorithm will be automatically re-hashed with the new algorithm when the user next logs in.

Values in the `password_algorithm` column are based on the PHC string format (https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md). Discourse's existing algorithm is described by the string `$pbkdf2-sha256$i=64000,l=32$`

To introduce a new algorithm and start using it, make sure it's implemented in the `PasswordHasher` library, then update `User::TARGET_PASSWORD_ALGORITHM`.

lib/password_hasher.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+class PasswordHasher
+  class InvalidAlgorithmError < StandardError
+  end
+
+  class UnsupportedAlgorithmError < StandardError
+  end
+
+  HANDLERS = {}
+
+  def self.register_handler(id, &blk)
+    HANDLERS[id] = blk
+  end
+
+  # Algorithm should be specified according to the id/params parts of the
+  # PHC string format.
+  # https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md
+  def self.hash_password(password:, salt:, algorithm:)
+    algorithm = algorithm.delete_prefix("$").delete_suffix("$")
+
+    parts = algorithm.split("$")
+    raise InvalidAlgorithmError if parts.length != 2
+
+    algorithm_id, algorithm_params = parts
+
+    algorithm_params = algorithm_params.split(",").map { |pair| pair.split("=") }.to_h
+
+    handler = HANDLERS[algorithm_id]
+    if handler.nil?
+      raise UnsupportedAlgorithmError.new "#{algorithm_id} is not a supported password algorithm"
+    end
+
+    handler.call(password: password, salt: salt, params: algorithm_params)
+  end
+
+  register_handler("pbkdf2-sha256") do |password:, salt:, params:|
+    raise ArgumentError.new("Salt and password must be supplied") if password.blank? || salt.blank?
+
+    if params["l"].to_i != 32
+      raise UnsupportedAlgorithmError.new("pbkdf2 implementation only supports l=32")
+    end
+
+    if params["i"].to_i < 1
+      raise UnsupportedAlgorithmError.new("pbkdf2 iterations must be 1 or more")
+    end
+
+    Pbkdf2.hash_password(password, salt, params["i"].to_i, "sha256")
+  end
+end