chore: CON-1682 Rename VetKdPayload to ChainKeyPayload

[eichhorl]

Background

The online phases of the VetKd and tECDSA/tSchnorr protocols are very similar: for each request context, combine and include all validated shares into a block. For historical reasons, the tECDSA/tSchnorr implementation is part of the IDKG payload, whereas the VetKd implementation is part of the batch payload.

Using the batch payload (over the IDKG payload) also for tECDSA/tSchnorr comes with the following advantages:

• Maximum payload size limitation out of the box
• No more incompatibility issues, since there is no batch payload in summary blocks
• Established location for user data/requests in the block
• Less code duplication by re-using VetKD implementation

For these reasons, this PR begins the process of generalizing the VetKD batch payload, such that it can also hold tECDSA/tSchnorr agreements in the future.

Proposed Changes

In particular, this PR renames the following types:

• VetKdPayload.* -> ChainKeyPayload.*
• VetKdAgreement -> ChainKeyAgreement
• VetKdErrorCode -> ChainKeyErrorCode
• vetkd_payload_to_bytes -> chain_key_payload_to_bytes
• bytes_to_vetkd_payload -> bytes_to_chain_key_payload

Additionally, the following metrics were renamed:

• vetkd_payload_build_duration -> chain_key_payload_build_duration
• vetkd_payload_metrics -> chain_key_payload_metrics
• vetkd_payload_errors -> chain_key_payload_errors

The alerts and dashboards corresponding to these metrics will be updated in a follow up.