[tls] Add an extension point for TLS handshaker behavior. (#12658)

Additional Description: This PR necessitated decoupling SslHandshakerImpl from ContextConfig a bit. We now pass an int representing the index of the extended_info struct rather than the ContextConfig.

This PR moves SslHandshakerImpl to its own build target, moves SslHandshaker construction into the ContextConfig, and adds a HandshakerFactoryContext and HandshakerFactory for modifying the ContextConfig's behavior when constructing a Handshaker. This PR also adds a control (requireCertificates) to turn off the release asserts that a context must have certificates.

This PR builds off work in https://github.com/envoyproxy/envoy/pull/12571 and refines work done (and abandoned) in https://github.com/envoyproxy/envoy/pull/12075/. For more discussion please see the comments section of https://github.com/envoyproxy/envoy/pull/12075/.

Risk Level: Low. This PR does not modify existing handshaking behavior, it just adds an extension point for modifying it.
Testing: A representative alternative implementation was added under :handshaker_test.
Docs Changes: N/a
Release Notes: N/a

Signed-off-by: James Buckland <jbuckland@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 7d6e7a4e559bdf0346687f7f404412e2412ea6fb

envoy/extensions/transport_sockets/tls/v3/tls.proto

@@ -99,7 +99,7 @@ message DownstreamTlsContext {
 }
 
 // TLS context shared by both client and server TLS contexts.
-// [#next-free-field: 13]
+// [#next-free-field: 14]
 message CommonTlsContext {
   option (udpa.annotations.versioning).previous_message_type = "envoy.api.v2.auth.CommonTlsContext";
 
@@ -238,4 +238,8 @@ message CommonTlsContext {
   //
   // There is no default for this parameter. If empty, Envoy will not expose ALPN.
   repeated string alpn_protocols = 4;
+
+  // Custom TLS handshaker. If empty, defaults to native TLS handshaking
+  // behavior.
+  config.core.v3.TypedExtensionConfig custom_handshaker = 13;
 }

envoy/extensions/transport_sockets/tls/v4alpha/tls.proto

@@ -98,7 +98,7 @@ message DownstreamTlsContext {
 }
 
 // TLS context shared by both client and server TLS contexts.
-// [#next-free-field: 13]
+// [#next-free-field: 14]
 message CommonTlsContext {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.extensions.transport_sockets.tls.v3.CommonTlsContext";
@@ -243,4 +243,8 @@ message CommonTlsContext {
   //
   // There is no default for this parameter. If empty, Envoy will not expose ALPN.
   repeated string alpn_protocols = 4;
+
+  // Custom TLS handshaker. If empty, defaults to native TLS handshaking
+  // behavior.
+  config.core.v4alpha.TypedExtensionConfig custom_handshaker = 13;
 }