verify cluster name of TLS peer certificates#52128
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OSS backport of https://github.com/gravitational/teleport-private/pull/1787.
This fixes https://github.com/gravitational/teleport-private/issues/1749 by verifying that the cluster name of a TLS peer cert matches the cluster name of the CA that issued it. Checks have been also added to ensure all locations in a CA where the cluster name is specified all match. These checks together ensure that CAs provided by trusted leaf clusters cannot sign a cert that will be accepted by the root as if it belongs to the root cluster.
You can't really get useful info of the certs in a *x509.CertPool, so I modified logic that builds client CA cert pools to return the information I need to do the additional checks alongside the cert pool itself. I lookup the CA cert that issued the client TLS cert by the cert's serial number, this means if CAs contain trusted certs that share serial numbers these checks could be bypassed. This shouldn't happen normally, I would think this would likely only happen if a malicious leaf cluster somehow knew the serial number of a root Host or User CA but didn't have the CA itself (or else better attacks could be done).
ClientCertPool was removed from middleware.go because it is an unused duplicate func likely left there by accident.
Tested by (on root and leaf clusters):
Connecting via tsh and the Web UI
Connecting to an App
Connecting to an SSH
Connecting to a database
changelog: verify that cluster name of TLS peer certs matches the cluster name of the CA that issued it to prevent Auth bypasses