Define methods to obtain properties of the interfaces

[harlem88]

• property( interface: str, path: str) -> value
• interface_props( interface: str ) -> [{ value, path }]
• all_props() -> [{interface, [{ value, path }]}]

[rgallor]

Thinking about the new methods, It could be useful to return the interface (major) version and ownership, to simplify further checks on the returned values.

In idea of the possible implementation could be the following:

• all_props() -> AllProps
• interface_props( interface: str) -> Interface
• property( interface: str, path: str) -> AstarteValue

where:

message AllProps {
     map<string, Interface> values = 1;
}

message Interface {
     Ownership ownership = 1;
     int32 version_major = 2;
     repeated StoredProp = 3;
}

message Ownership {
    Device = 1;
    Server = 2;
}

message StoredProp {
     string path = 1;
     AstarteValue value = 2;
}