model: Add string serialization/deserialization of Reference objects according to DotAAS sec. 5.2.1

In Details of the Asset Administration Shell, section 5.2.1 (p. 95), a cannonical string representation of Reference objects is presented:

In some mapping or serializations the Type “Reference” is converted into a single string. In this case we recommend to use the following serilization:

<Reference> ::= <Key>{,<Key>}*
<Key> ::= (<KeyType>)(<Local>)[<KeyIdType>]<KeyValue>
<KeyType> ::= value of AAS:Key/type
<Local> ::= local | no-local
<KeyIdType> ::= value of AAS:Key/.idType
<KeyValue> ::= value of AAS:Key/value

With <Local> == local if AAS:Key/local = True and no-local if AAS:Key/local == False.

Examples:

• (ConceptDescription)(local)[IRDI]0173-1#02-BAA120#008
• (GlobalReference)(no-local)[IRDI]0173-1#01-AFZ615#016
• (Submodel)(local)[IRI]http://customer.com/demo/aas/1/1/1234859590,(Property)(local)[IdShort]Temperature

(Plattform Industrie 4.0: "Details of the Asset Administration Shell – Part 1 - The exchange of information between partners in the value chain of Industrie 4.0 (Version 2.0.1)", CC BY-ND 4.0)

We could perfectly implement this representation as the __str__ method of the Reference class and add a parser/constructor method like this:

@classmethod
def from_string(cls, value: str) -> "Reference":
    # probably something with regex parsing here