Update indicator validation schema

[fdiblen]

Description: Updates the validation schema of the indicators.
Changes:

• created field is not required
• updated author field to add email and url
• update contact field: add @type, email, name
• change source field type to array as there can be multiple sources (see Create dependency management indicator #23)
• update source field: add @id, identifier, @type, url, name
• adapt (make array) source field of
  • indicators/no_leaked_credential.json
  • indicators/no_critical_vulnerability.json

Ref:

• Fix validation of indicators #37
• Create dependency management indicator #23