[New] Add XAHAU support

[WietseWind]

Hi @KevinKoobs,

Another Todo done:

I added XAHAU activation support to the backend.

Meaning you can now query for Xahau specific activation eligibility. If unspecified, mainnet is assumed.

What you do is: add /network:xahau/ to the URL.

E.g.:
https://xapps.xumm.app/activate/check-claim/serial:AB02000000055357/network:xahau

Same applies to committing the activation, and fetching results. So all variants of the endpoint.

This means the xActivate xApp can be updated to check for prefunding/activation eligibility & activation on Xahau as well 🎉