fix(adapter): require the __before_compile__/1 callback

`Nebulex.Adapter` listed `__before_compile__/1` as optional, but
`Nebulex.Cache` registers `@before_compile adapter` unconditionally, so an
adapter without it fails to compile any cache that uses it. Drop the
optional marker so the behaviour states the actual requirement and the
compiler reports a missing implementation on the adapter itself.

Author: cabol

CHANGELOG.md

@@ -27,6 +27,9 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Bug fixes
 
+- [Nebulex.Adapter] `__before_compile__/1` is no longer listed as an optional
+  callback. `Nebulex.Cache` has always invoked it, so an adapter without it
+  could not be used by any cache.
 - [Nebulex.Adapter] `run_command/4` now forwards the shared `:telemetry`,
   `:telemetry_event`, and `:telemetry_metadata` options to adapter
   callbacks. Nested commands receiving these options respect

lib/nebulex/adapter.ex

@@ -44,9 +44,6 @@ defmodule Nebulex.Adapter do
   """
   @callback init(config :: keyword()) :: {:ok, :supervisor.child_spec(), adapter_meta()}
 
-  # Define optional callbacks
-  @optional_callbacks __before_compile__: 1
-
   ## API
 
   # Inline common instructions