Memoize key_provider from key or deterministic key_provider if any

The memoization of Scheme#key_provider was removed completely in
rails#51019 because it prevented
overriding the `key_provider` via `with_encryption_context`. However,
this also made our tests for the HEY app about 5 times slower. We traced
this down to all attributes where we either provide a key or declare them
as deterministic and have to derive a key to instantiate the provider
every time we load them. This might happen hundreds of times per test,
and ultimately, we call
```
ActiveSupport::KeyGenerator#generate_key
```
and
```
OpenSSL::KDF.pbkdf2_hmac
```
hundreds of times. This adds significant overhead per test.

In reality, what's overridden bv `with_encryption_context` is the value
used as default provider, that is, `ActiveRecord::Encryption.key_provider`.
This is only used in `Scheme#key_provider` if the scheme doesn't already have
either a `key_provider` passed directly, or a `key`, or is `deterministic`.
The `key_provider` passed directly is already memoized simply by having it
stored as is. Let's memoize the other two, in order, so we save all those
extra calls to derive the same keys again and again.

Author: rosa

activerecord/lib/active_record/encryption/scheme.rb

@@ -50,7 +50,7 @@ def fixed?
       end
 
       def key_provider
-        @key_provider_param || build_key_provider || default_key_provider
+        @key_provider_param || key_provider_from_key || deterministic_key_provider || default_key_provider
       end
 
       def merge(other_scheme)
@@ -80,10 +80,14 @@ def validate_config!
           raise Errors::Configuration, "key_provider: and key: can't be used simultaneously" if @key_provider_param && @key
         end
 
-        def build_key_provider
-          return DerivedSecretKeyProvider.new(@key) if @key.present?
+        def key_provider_from_key
+          @key_provider_from_key ||= if @key.present?
+            DerivedSecretKeyProvider.new(@key)
+          end
+        end
 
-          if @deterministic
+        def deterministic_key_provider
+          @deterministic_key_provider ||= if @deterministic
             DeterministicKeyProvider.new(ActiveRecord::Encryption.config.deterministic_key)
           end
         end