feat: add CredentialStore + inmem implementation

[paullatzelsperger]

What this PR changes/adds

This PR adds a CredentialStore interface plus an In-Memory implementation for it, which is the default.

Why it does that

Subsequent work items (#153, #154) will depend on a storage backend for VerifiableCredentials.

Further notes

• this store really is just a CRUD backend, more elaborate operations like composing a query "by type" have to be done in other layers.
• added some dummy/NOOP services, just so the launcher is able to start. those will get replaced with actual services in IATP: Implement CredentialQueryResolver #153 and IATP: Implement PresentationGenerator module #154.
• fixed minor bugs in the validator and the transformer
• added the identity-did-web module to the launcher

Linked Issue(s)

Closes #163

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