feat: add token_wallet module to allow funding IOTA DIDs

[daniel-mader]

Description of change

With the new module token_wallet a funding address is introduced that can receive IOTA tokens and put down the storage deposit when a non-published IOTA document is produced and published. On destruction of the DID document, the deposit is returned to the funding address and can be sent to a new receiving address (such as a crypto wallet).

Additionally, most of the IOTA-related code was moved to a new did_iota module.

Links to any relevant issues

• Add light-weight wallet to manage IOTA tokens #7

Depends on

• refactor: use thiserror to handle errors, produce documents for did:jwk and did:key according to specifications #4
• feat: produce did:iota from a published DID document #11

How the change has been tested

n/a

Definition of Done checklist

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