UI: Expose MOD_MESHSEQ_READ_ATTRIBUTES flag for Mesh Sequence Cache modifier

There is an internal flag which gets set only during USD import,
which allows for custom attributes to be read from Mesh Sequence Cache.
This flag is not shown anywhere in the UI, and is not accessible through
python, which can lead to confusing behavior.

For example, loading a .usd file through `bpy.ops.wm.usd_import` will read
custom attributes, but when  `Mesh Sequence Cache` is added manually then
loading the same usd file through `bpy.ops.cachefile.open` will not load
custom attributes.

Further, if the user edits `object.modifiers['MeshSequenceCache'].read_data`
then `MOD_MESHSEQ_READ_ATTRIBUTES` flag will be removed with no way
to bring it back.

Pull Request: https://projects.blender.org/blender/blender/pulls/132475

source/blender/makesrna/intern/rna_modifier.cc

@@ -6981,6 +6981,7 @@ static void rna_def_modifier_meshseqcache(BlenderRNA *brna)
       {MOD_MESHSEQ_READ_POLY, "POLY", 0, "Faces", ""},
       {MOD_MESHSEQ_READ_UV, "UV", 0, "UV", ""},
       {MOD_MESHSEQ_READ_COLOR, "COLOR", 0, "Color", ""},
+      {MOD_MESHSEQ_READ_ATTRIBUTES, "ATTRIBUTES", 0, "Attributes", ""},
       {0, nullptr, 0, nullptr, nullptr},
   };