feat: use IOTA Sponsoring Service to fund did:iota transactions

[nanderstabel]

Description of change

• Adds iota_sponsoring_service and iota_sponsoring_service_auth config variable that can be used to (pre) configure the client for the IOTA Sponsoring Service.
• Adds /.well-known/sponsoring-configuration endpoint which exposes data (most importantly iota wallet address) that IOTA Sponsoring Service can use to register UniCore as a client.

Edit:

• changed iota_sponsoring_service ==> iota_sponsoring_service_url
• changed /.well-known/sponsoring-configuration ==> /public/sponsoring-configuration

Links to any relevant issues

n/a

How the change has been tested

Can be tested against the Moveathon IOTA Sponsoring Service demo deployment.

Definition of Done checklist

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• I have commented my code, particularly in hard-to-understand areas
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• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
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