LibAudio: Generalize an encoder interface

This interface is very simple for the time being and can be used to
provide encoding functionality in a generalized way. Initialization and
parameter setting are intentionally not abstracted for now, since this
is usually very format-specific. We just need a general interface for
writing samples and errorable finalization.

Userland/Libraries/LibAudio/Encoder.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/Span.h>
+#include <LibAudio/Sample.h>
+
+namespace Audio {
+
+class Encoder {
+public:
+    virtual ~Encoder() = default;
+
+    // Encodes the given samples and writes them to the output stream.
+    // Note that due to format restrictions, not all samples might be written immediately, this is only guaranteed after a call to finalize().
+    virtual ErrorOr<void> write_samples(ReadonlySpan<Sample> samples) = 0;
+
+    // Finalizes the stream, future calls to write_samples() will cause an error.
+    // This method makes sure that all samples are encoded and written out.
+    // This method is called in the destructor, but since this can error, you should call this function yourself before disposing of the decoder.
+    virtual ErrorOr<void> finalize() = 0;
+};
+
+}