alt_svc_cache: adding flush capabilities (#18189)

Risk Level: low
Testing: new unit tests. integration test TODO in a follow up
Docs Changes: n/a
Release Notes: will land with integration test.
Fixes #18034

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ 7f63632be6b08193550041c4069e63f557b16c5d

envoy/config/core/v3/protocol.proto

@@ -102,6 +102,12 @@ message AlternateProtocolsCacheOptions {
   //   it is possible for the maximum entries in the cache to go slightly above the configured
   //   value depending on timing. This is similar to how other circuit breakers work.
   google.protobuf.UInt32Value max_entries = 2 [(validate.rules).uint32 = {gt: 0}];
+
+  // Allows configuring a persistent
+  // :ref:`key value store <envoy_v3_api_msg_config.common.key_value.v3.KeyValueStoreConfig>` to flush
+  // alternate protocols entries to disk.
+  // This function is currently only supported if concurrency is 1
+  TypedExtensionConfig key_value_store_config = 3;
 }
 
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