show child assets in the asset page

[victorgarcia98]

Description

This PR introduces a new section to the asset page to list all the child assets of a given asset.

Look & Feel

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• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on code under GPL or other license that is incompatible with FlexMeasures

[nhoening]

I see you circumvented the internal API as it doesn't support this. I remember you talking about that. Maybe add a comment on the why.
It's a good thing that the internal API gets called before for the parent asset, which will help with auth protection. This does matter.

[victorgarcia98]

I see you circumvented the internal API as it doesn't support this. I remember you talking about that. Maybe add a comment on the why. It's a good thing that the internal API gets called before for the parent asset, which will help with auth protection. This does matter.

Finally, I adapted the internal API to return the children (at least, the those of the first level).

[nhoening]

You found a good solution here (regarding the child asset issue). Great job!