Optimize processing of slashings (#14990)

* Calculate max epoch and churn for slashing once

* calculate once for proposer and attester slashings

* changelog <3

* introduce struct

* check if err is nil in ProcessVoluntaryExits

* rename exitData to exitInfo and return from functions

* cleanup + tests

* cleanup after rebase

* Potuz's review

* pre-calculate total active balance

* remove `slashValidatorFunc` closure

* Avoid a second validator loop

    🤖 Generated with [Claude Code](https://claude.ai/code)

    Co-Authored-By: Claude <noreply@anthropic.com>

* remove balance parameter from slashing functions

---------

Co-authored-by: terence tsao <terence@prysmaticlabs.com>
Co-authored-by: potuz <potuz@prysmaticlabs.com>

api/server/httprest/options.go

@@ -1,9 +1,8 @@
 package httprest
 
 import (
-	"time"
-
 	"net/http"
+	"time"
 
 	"github.com/OffchainLabs/prysm/v6/api/server/middleware"
 )