Use enumer to generate String() methods for most enums (#25705)

We have many hand-written String() methods (and similar) for enums.
These require more maintenance and are more error-prone than using
automatically generated methods. In addition, the auto-generated
versions can be more efficient.

Here, we switch to using https://github.com/loggerhead/enumer, itself
a fork of https://github.com/diegostamigni/enumer, no longer maintained,
and a fork of the mostly standard tool
https://pkg.go.dev/golang.org/x/tools/cmd/stringer.
We use this fork of enumer for Go 1.20+ compatibility and because
we require the `-transform` flag to be able to generate
constants that match our current code base.

Some enums were not targeted for this change:

sdk/logical/managed_key.go

@@ -11,6 +11,7 @@ import (
 	wrapping "github.com/hashicorp/go-kms-wrapping/v2"
 )
 
+//go:generate enumer -type=KeyUsage -trimprefix=KeyUsage -transform=snake
 type KeyUsage int
 
 const (