Flatten compression algorithm setting (#8265)

This flattens the compression algorithm setting, removing the
`Option<_>` wrapping layer and making handling of the setting easier.

It also adds a specific setting for *disabled* compression with the
continued ability to read copmressed data, giving us the option to
more easily back out of a compression rollout, should the need arise,
which was one of the limitations of #8238.

Implements my suggestion from
https://github.com/neondatabase/neon/pull/8238#issuecomment-2206181594 ,
inspired by Christian's review in
https://github.com/neondatabase/neon/pull/8238#pullrequestreview-2156460268 .

Part of #5431

libs/pageserver_api/src/models.rs

@@ -450,9 +450,22 @@ pub enum CompactionAlgorithm {
 )]
 #[strum(serialize_all = "kebab-case")]
 pub enum ImageCompressionAlgorithm {
+    /// Disabled for writes, and never decompress during reading.
+    /// Never set this after you've enabled compression once!
+    DisabledNoDecompress,
+    // Disabled for writes, support decompressing during read path
+    Disabled,
     /// Zstandard compression. Level 0 means and None mean the same (default level). Levels can be negative as well.
     /// For details, see the [manual](http://facebook.github.io/zstd/zstd_manual.html).
-    Zstd { level: Option<i8> },
+    Zstd {
+        level: Option<i8>,
+    },
+}
+
+impl ImageCompressionAlgorithm {
+    pub fn allow_decompression(&self) -> bool {
+        !matches!(self, ImageCompressionAlgorithm::DisabledNoDecompress)
+    }
 }
 
 #[derive(Eq, PartialEq, Debug, Clone, Serialize, Deserialize)]