feat: add VDR_SIGNING_KEY to the DIDOperations

[yshyn-iohk]

Overview

The VDR signing key was added to the DIDOperations.
Implements #882

Checklists

Pre-submit checklist:

• Self-reviewed the diff
• New code has proper comments/documentation/tests
• Any changes not covered by tests have been tested manually
• The README files are updated
• If new libraries are included, they have licenses compatible with our project
• If there is a db migration altering existing tables, there is a proper migration test

Pre-merge checklist:

• Commits have useful messages
• Review clarifications made it into the code

[github-actions]

Unit Test Results

293 tests  +5   293 ✔️ +5   20s ⏱️ -1s
  36 suites ±0       0 💤 ±0 
  36 files   ±0       0 ❌ ±0 

Results for commit 41aa193. ± Comparison against base commit 301093e.

♻️ This comment has been updated with latest results.

[atala-dev]

🦙 MegaLinter status: ⚠️ WARNING

Descriptor	Linter	Files	Fixed	Errors
⚠️ PROTOBUF	protolint	1		1
✅ REPOSITORY	dustilock	yes		no
✅ REPOSITORY	git_diff	yes		no
⚠️ REPOSITORY	kics	yes		1
✅ REPOSITORY	syft	yes		no
✅ SQL	sql-lint	1		0

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