feat: Add custom module type support

[kuzznya]

This is a suggestion to add support for custom module type parsing by allowing the user of a library to implement it.

Example usage:

const myType = "mytypeid"

func parse() {
	decoder := core.NewDecoder(r).WithSpecialType(ftsIndexType, parseMyType)
}

func parseMyType(h core.ModuleTypeHandler, encVersion int) (interface{}, error) {
	opcode, err := h.ReadOpcode()
	if err != nil {
		return nil, err
	}
	if opcode != core.OpcodeString {
		return nil, errors.New("unsupported module type value code")
	}

	data, err := h.ReadString()
	if err != nil {
		return nil, err
	}

	val := model.MyModel{}
	err = json.Unmarshal(data, &val)
	if err != nil {
		return nil, err
	}

	opcode, err = h.ReadOpcode()
	if err != nil || opcode != core.OpcodeEOF {
		return nil, errors.New("expected module type value EOF")
	}

	return &val, nil
}

This implementation was used in my fork for my project, but it would be nice to have it supported in your library.

I'm open to any comments about the design of the solution and ready to update the PR in my free time if anything required.

[kuzznya]

I can write the tests once the approach itself would be approved

[HDT3213]

Actually, I think it is a good idea. Please add some tests so I can merge it into master

[kuzznya]

Hi @HDT3213, I've created the test similar to existing ones. Failed to reach the same coverage results, so if required, I can create some more, but at least main case is covered