Integrate state-diff into State() (#16033)

**What type of PR is this?**
Feature

**What does this PR do? Why is it needed?**
This PR integrates the state diff path into the `State()` function from
`db/kv`, which allows reading of states using the state diff db, when
the `EnableStateDiff` flag is enabled.

**Notes for reviewers:**
Files `kv/state_diff_test.go` and `config/features/config.go` only
contain renamings:
- `kv/state_diff_test.go`: rename `setDefaultExponents()` to
`setDefaultStateDiffExponents()` to be less vague.
- `config/features/config.go`: rename `enableStateDiff` to
`EnableStateDiff` to make it public.

config/features/config.go

@@ -286,8 +286,8 @@ func ConfigureBeaconChain(ctx *cli.Context) error {
 		cfg.DisableLastEpochTargets = true
 	}
 
-	if ctx.IsSet(enableStateDiff.Name) {
-		logEnabled(enableStateDiff)
+	if ctx.IsSet(EnableStateDiff.Name) {
+		logEnabled(EnableStateDiff)
 		cfg.EnableStateDiff = true
 
 		if ctx.IsSet(enableHistoricalSpaceRepresentation.Name) {

config/features/flags.go

@@ -172,7 +172,7 @@ var (
 		Name:  "enable-experimental-attestation-pool",
 		Usage: "Enables an experimental attestation pool design.",
 	}
-	enableStateDiff = &cli.BoolFlag{
+	EnableStateDiff = &cli.BoolFlag{
 		Name:  "enable-state-diff",
 		Usage: "Enables the experimental state diff feature.",
 	}