crypto/tls: add ExampleConfig_VerifyPeerCertificate
crypto/tls: add ExampleConfig_VerifyPeerCertificate

Setting InsecureSkipVerify and VerifyPeerCertificate is the recommended
way to customize and override certificate validation.

However, there is boilerplate involved and it usually requires first
reimplementing the default validation strategy to then customize it.
Provide an example that does the same thing as the default as a starting
point.

Examples of where we directed users to do something similar are in
issues #35467, #31791, #28754, #21971, and #24151.

Fixes #31792

Change-Id: Id033e9fa3cac9dff1f7be05c72dfb34b4f973fd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/193620
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

cloudflare/go,akutz/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,akutz/go,akutz/go,cloudflare/go,akutz/go,nawawi/go,golang/go,akutz/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,golang/go,CAFxX/go,golang/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,CAFxX/go,CAFxX/go