crypto/tls: add HandshakeContext method to Conn

Adds the (*tls.Conn).HandshakeContext method. This allows
us to pass the context provided down the call stack to
eventually reach the tls.ClientHelloInfo and
tls.CertificateRequestInfo structs.
These contexts are exposed to the user as read-only via Context()
methods.

This allows users of (*tls.Config).GetCertificate and
(*tls.Config).GetClientCertificate to use the context for
request scoped parameters and cancellation.

Replace uses of (*tls.Conn).Handshake with (*tls.Conn).HandshakeContext
where appropriate, to propagate existing contexts.

Fixes #32406

Change-Id: I33c228904fe82dcf57683b63627497d3eb841ff2
Reviewed-on: https://go-review.googlesource.com/c/go/+/246338
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

common.go

@@ -7,6 +7,7 @@ package tls
 import (
 	"bytes"
 	"container/list"
+	"context"
 	"crypto"
 	"crypto/ecdsa"
 	"crypto/ed25519"
@@ -444,6 +445,16 @@ type ClientHelloInfo struct {
 	// config is embedded by the GetCertificate or GetConfigForClient caller,
 	// for use with SupportsCertificate.
 	config *Config
+
+	// ctx is the context of the handshake that is in progress.
+	ctx context.Context
+}
+
+// Context returns the context of the handshake that is in progress.
+// This context is a child of the context passed to HandshakeContext,
+// if any, and is canceled when the handshake concludes.
+func (c *ClientHelloInfo) Context() context.Context {
+	return c.ctx
 }
 
 // CertificateRequestInfo contains information from a server's
@@ -462,6 +473,16 @@ type CertificateRequestInfo struct {
 
 	// Version is the TLS version that was negotiated for this connection.
 	Version uint16
+
+	// ctx is the context of the handshake that is in progress.
+	ctx context.Context
+}
+
+// Context returns the context of the handshake that is in progress.
+// This context is a child of the context passed to HandshakeContext,
+// if any, and is canceled when the handshake concludes.
+func (c *CertificateRequestInfo) Context() context.Context {
+	return c.ctx
 }
 
 // RenegotiationSupport enumerates the different levels of support for TLS