Add initial data structure for dist-manifest.json

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

crates/binstalk-fetchers/src/dist_manifest_fetcher.rs

@@ -0,0 +1,51 @@
+use std::collections::BTreeMap;
+
+use binstalk_downloader::remote::{Client, Response};
+use cargo_dist_schema::Format;
+use compact_str::CompactString;
+
+use crate::FetchError;
+
+#[derive(Clone, Debug)]
+struct Binary {
+    /// Key: target, value: artifact
+    artifacts: BTreeMap<CompactString, Artifact>,
+}
+
+/// An tarball/zip artifact.
+#[derive(Clone, Debug)]
+struct Artifact {
+    /// Filename of artifact on release artifacts,
+    /// need to infer the format.
+    filename: CompactString,
+    /// Path to the executable within the tarbal/zip.
+    path_to_exe: CompactString,
+
+    /// Filename of the checksum file.
+    checksum_filename: Option<CompactString>,
+}
+
+#[derive(Clone, Debug)]
+pub(super) enum DistManifest {
+    NotSupported(Format),
+    /// Key: name of the binary
+    Binaries(BTreeMap<CompactString, Binary>),
+}
+
+impl DistManifest {
+    async fn parse(response: Response) -> Result<Self, FetchError> {
+        let manifest: cargo_dist_schema::DistManifest = response.json().await?;
+        let format = manifest.format();
+
+        if format.unsupported() {
+            return Ok(Self::NotSupported(format));
+        }
+
+        todo!()
+    }
+}
+
+// TODO: Cache `DistManifest` in a new global http cacher for the fetchers
+// Also cache the artifacts downloaded and extracted
+
+pub struct GhDistManifest {}