Respect DID PLC constraints

[johnbillion]

There appears to be some constraints present in the DID PLC spec:

• rotationKeys: "at most 5 keys, with no duplication".
• verificationMethods: "A total limit of 10 verificationMethods (per DID) has been added", although this constraint only appears in the changelog at the bottom, not in the spec itself. Needs confirming.
• Operational constraints in https://github.com/did-method-plc/did-method-plc/blob/main/packages/server/src/constraints.ts