API endpoint for last_registered_epoch (or similar)

**Is your feature request related to a problem? Please describe.**

Currently, there is no way to check that a validator is active via the relay API. You can see if an initial registration was successful via the `relay/v1/data/validator_registration` API call but if a validator goes inactive after registering there is no easy way to tell.

For example, this goerli validator is inactive at the time of this query:

API call:
```
https://0xafa4c6985aa049fb79dd37010438cfebeb0f2bd42b115b89dd678dab0670c1de38da0c4e9138c9290a398ecd9a0b3110@boost-relay-goerli.flashbots.net/relay/v1/data/validator_registration?pubkey=0xaa0508f2cbd370a6e92fc37ef12709dab2b2a6d8d267db4e7e3828d2b4485c2ff6c83875b5bb93eca8867c390c0100b8
```
Response:
```
{
"message": {
"fee_recipient": "0x6a7aa9b882d50bb7bc5da1a244719c99f12f06a3",
"gas_limit": "30000000",
"timestamp": "1664138161",
"pubkey": "0xaa0508f2cbd370a6e92fc37ef12709dab2b2a6d8d267db4e7e3828d2b4485c2ff6c83875b5bb93eca8867c390c0100b8"
},
"signature": "0x8bd041544e396108730e9796f76ab74a643bda3d5cb9927d76b7f686ed94e2fac359ba9fd9211834069ed9686b6d1d961457c385c011b2515b3eddcec2a8f635315d7c3732e42eae34deb802caaf5d721a4720e7bf0c86f9a39017765979b996"
}
```
The timestamp in the message is cached for good reason (new signatures+verification every Epoch does not make sense). This makes it impossible to see if a new registration was successful because the timestamp will not change without changing the `fee_recipient` or some other data.

**Describe the solution you'd like**

It would be nice to have a `last_active_epoch` data point that is either returned from this API call (must be ourside of the "message" since that carries a signature) or a separate, new endpoint (possibly something like `/relay/v1/data/validator_last_active_epoch` or `/relay/v1/data/validator_last_registered_epoch`). This metric would have significantly less overhead than updating the timestamp but would still need to be tracked internally by validators.

**Describe alternatives you've considered**

Updating the timestamp to the most recent epoch for which the validator registered. This does not make since because the message is signed by the validator which currently cache the message to prevent 400K signature verifications by the relay on each Epoch.

**Additional context**

If there is consensus on the correct path forward I am happy to implement the changes.