Currently, there is no way to cancel or put a deadline on certificate verification operations, which due to the graph nature of chain building (and the associated signature verifications) can be somewhat expensive.

I propose we add a context.Context to the VerifyOptions struct, which would then be checked during path building. This can then be populated during TLS handshakes using the existing context.

type VerifyOptions struct {
	...

	// Context is the context used for certificate chain building and
	// verification. If nil, verification time is unbounded.
	Context context.Context
}

cc @FiloSottile