Add support for reading and writing compressed blobs (#8106)

Add support for reading and writing zstd-compressed blobs for use in
image layer generation, but maybe one day useful also for delta layers.
The reading of them is unconditional while the writing is controlled by
the `image_compression` config variable allowing for experiments.

For the on-disk format, we re-use some of the bitpatterns we currently
keep reserved for blobs larger than 256 MiB. This assumes that we have
never ever written any such large blobs to image layers.

After the preparation in #7852, we now are unable to read blobs with a
size larger than 256 MiB (or write them).

A non-goal of this PR is to come up with good heuristics of when to
compress a bitpattern. This is left for future work.

Parts of the PR were inspired by #7091.

cc  #7879

Part of #5431

libs/pageserver_api/src/models.rs

@@ -432,6 +432,24 @@ pub enum CompactionAlgorithm {
     Tiered,
 }
 
+#[derive(
+    Debug,
+    Clone,
+    Copy,
+    PartialEq,
+    Eq,
+    Serialize,
+    Deserialize,
+    strum_macros::FromRepr,
+    strum_macros::EnumString,
+)]
+#[strum(serialize_all = "kebab-case")]
+pub enum ImageCompressionAlgorithm {
+    /// 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> },
+}
+
 #[derive(Eq, PartialEq, Debug, Clone, Serialize, Deserialize)]
 pub struct CompactionAlgorithmSettings {
     pub kind: CompactionAlgorithm,