Factor out a LogStore interface

Continue progress on #5729.

Just as I hoped, this uncovered an issue: the daemon protocol is missing
a way to query build logs. This doesn't effect `unix://`, but does
effect `ssh://`. A FIXME is left for this, so we come back to it later.

src/libstore/log-store.hh

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "store-api.hh"
+
+
+namespace nix {
+
+struct LogStore : public virtual Store
+{
+    /* Return the build log of the specified store path, if available,
+       or null otherwise. */
+    virtual std::optional<std::string> getBuildLog(const StorePath & path) = 0;
+
+    virtual void addBuildLog(const StorePath & path, std::string_view log) = 0;
+
+    static LogStore & require(Store & store);
+};
+
+}