nix hash path, and preperatory refactors

- `nix store add` supports text hashing

  With functional test ensuring it matches `builtins.toFile`.

- Factored-out flags for both commands

- Move all common reusable flags to `libcmd`

  - They are not part of the *definition* of the CLI infra, just a usag
    of it.

  - The `libstore` flag couldn't go in `args.hh` in libutil anyways,
    would be awkward for it to live alone

- Shuffle around `Cmd*` hierarchy so flags for deprecated commands don't
  end up on the new ones

src/libcmd/misc-store-flags.hh

@@ -0,0 +1,21 @@
+#include "args.hh"
+#include "content-address.hh"
+
+namespace nix::flag {
+
+Args::Flag hashAlgo(std::string && longName, HashAlgorithm * ha);
+static inline Args::Flag hashAlgo(HashAlgorithm * ha)
+{
+    return hashAlgo("hash-algo", ha);
+}
+Args::Flag hashAlgoOpt(std::string && longName, std::optional<HashAlgorithm> * oha);
+Args::Flag hashFormatWithDefault(std::string && longName, HashFormat * hf);
+Args::Flag hashFormatOpt(std::string && longName, std::optional<HashFormat> * ohf);
+static inline Args::Flag hashAlgoOpt(std::optional<HashAlgorithm> * oha)
+{
+    return hashAlgoOpt("hash-algo", oha);
+}
+Args::Flag fileIngestionMethod(FileIngestionMethod * method);
+Args::Flag contentAddressMethod(ContentAddressMethod * method);
+
+}