Implement from_object/from_string method in Metadata class

[sechkova]

Description of feature request:
Implement a class method in Metadata class, similar to the existing from_file method but taking a file object as an argument.

See #1304 for context.

[jku]

I assume this is useful because client does not write new metadata to disk before it's verified so at that point there's only a file-like object but we want to instantiate the Metadata to verify it is correct?

I guess the other alternative to from_object() is from_string() (and client would then call from_string(tmp_file.read())). I don't think it's clearly better just thinking out loud.

[sechkova]

I assume this is useful because client does not write new metadata to disk before it's verified so at that point there's only a file-like object but we want to instantiate the Metadata to verify it is correct?

Yes, exactly.

I guess the other alternative to from_object() is from_string() (and client would then call from_string(tmp_file.read())). I don't think it's clearly better just thinking out loud.

You don't like a function signature with the vague definition of a file object? :)
String would be ok, raw bytes too, no need to be a file object with its associated methods.

Now that a separate serialisation was implemented, all we need is basically a method that calls deserialize.

[sechkova]

Something like the existing from_file but without the actual file read from the file system:

  @classmethod
   def from_string/from_bytes/from_raw(
       cls,
       raw_data: str/bytes,
       deserializer: Optional[MetadataDeserializer] = None
   ) -> "Metadata":

       if deserializer is None:
           deserializer = JSONDeserializer()

       return deserializer.deserialize(raw_data)

[jku]

looks good to me -- str seems to make sense as type to me