Add yaml parser workaround for anchors

[bastianjoel]

needed for OpenSlides/openslides-meta#69

[jsangmeister]

tested

[ostcar]

I am not a fan. I don't like, that the data it decoded, encoded again and decoded a second time.

I want to try something. If it doesn't work, we can do it this way

[jsangmeister]

I am not a fan. I don't like, that the data it decoded, encoded again and decoded a second time.

Yes, it's not optimal, but it's the best we could come up with. Currently, the parser is faced with two problems:

• There is no easy way to not parse the _meta key as a model, but as a separate attribute with a different interface
• The YAML lib does not handle anchors/aliases with different indentations correctly when using custom unmarshalers

This approach solves both of these problems in one go (pun not intended). If you're solution solves those as well in a better way, I would be happy to accept it.

[ostcar]

I was not able to find a better solution. So lets use it.

For the future, we could thing about changing the format of the models.yml so something like this:

meta: ...
config:...
other_fields: ...
collection:
motion:
...
agenda_item:
...

[bastianjoel]

We tried something similar but then stumbled over goccy/go-yaml#431