feat: Enable key rotation in MPTokenIssuanceSet

[yinyiqian1]

Pre-ConfidentialKeyRotation, MPTokenIssuanceSet rejects any attempt to modify an existing IssuerEncryptionKey/AuditorEncryptionKey, and an auditor key can never be provided without the issuer key (temMALFORMED in preflight).

Post-ConfidentialKeyRotation:

1. allow late-registering auditor key after issuance key already registered on ledger
2. prevent registering auditor key if issuance key is not already registered or not being registered in the same tx
3. allow late-registering auditor key even if COA > 0
4. allow rotating issuer key even if it already exists on ledger
5. allow rotating auditor key even if it already exists on ledger
6. allow rotating even if COA > 0

Epoch:

1. when not opt-in confidential, epoch absent
2. first time register key: epoch keeps absent, matching issuances whose keys were registered before the ConfidentialKeyRotation amendment.
3. rotating key: epoch + 1

High Level Overview of Change

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[oleks-rip]

Best practices for amendments are using one amendment enabled block and one amendment disabled block, even if they are similar, instead of spreading amendment through the code.

[oleks-rip]

These 2 have the same code, why not make helper. or lambda?