We need a way to modify how properties are stored in the host. The proposal is to add the following new function:
declare_property(property_name, const_type, scope_type, schema_type, schema)
where:

• property_name is the name of the property used in set_property and get_property, encoded the same way;
• const_type is one of readonly, mutable
• scope_type is one of filter_chain, downstream_request, and downstream_connection, controls the visibility of the property across modules
• schema_type is one of bytes, flatbuffers, allows computation on the property and traversing nested structures
• schema is an optional encoded schema (e.g. reflection flatbuffer)

First declaration wins. Declarations are optional, and default to some sane options.

cc @mandarjog @jplevyak