Add an `identifier` field to the `va.PerformValidationRequest` proto, which will soon replace its `dnsName` field.
Accept and prefer the `identifier` field in every VA function that uses this struct. Don't (yet) assume it will be present.
Throughout the VA, accept and handle the IP address identifier type. Handling is similar to DNS names, except that `getAddrs` is not called, and consider that:
- IPs are represented in a different field in the `x509.Certificate` struct.
- IPs must be presented as reverse DNS (`.arpa`) names in SNI for [TLS-ALPN-01 challenge requests](https://datatracker.ietf.org/doc/html/rfc8738#name-tls-with-application-layer-).
- IPv6 addresses are enclosed in square brackets when composing or parsing URLs.
For HTTP-01 challenges, accept redirects to bare IP addresses, which were previously rejected.
Fixes#2706
Part of #7311
3e6a8e2d25