Add DNS Server address override to the EventEngine API (#29485)

* Add DNS Server address override to the EventEngine API

* instead, override authority in GetDNSResolver; other fixes

* feedback

* constrain dns_server format to "IP:port"

doc/naming.md

@@ -46,11 +46,8 @@ Most gRPC implementations support the following URI schemes:
   - The name has no connection with filesystem pathnames.
   - No permissions will apply to the socket - any process/user may access the socket.
   - The underlying implementation of Abstract sockets uses a null byte ('\0')
-    as the first character; the implementation will prepend this null. Do not include 
+    as the first character; the implementation will prepend this null. Do not include
     the null in `abstract_path`.
-  - `abstract_path` cannot contain null bytes.
-    - TODO(https://github.com/grpc/grpc/issues/24638): Unix allows abstract socket names to contain null bytes, 
-      but this is not supported by the gRPC C-core implementation.
 
 The following schemes are supported by the gRPC C-core implementation,
 but may not be supported in other languages:

include/grpc/event_engine/event_engine.h

@@ -261,6 +261,12 @@ class EventEngine {
     struct LookupTaskHandle {
       intptr_t key[2];
     };
+    /// Optional configuration for DNSResolvers.
+    struct ResolverOptions {
+      /// If empty, default DNS servers will be used.
+      /// Must be in the "IP:port" format as described in naming.md.
+      std::string dns_server;
+    };
     /// DNS SRV record type.
     struct SRVRecord {
       std::string host;
@@ -293,7 +299,7 @@ class EventEngine {
     ///
     /// If cancelled, \a on_resolve will not be executed.
     virtual LookupTaskHandle LookupHostname(LookupHostnameCallback on_resolve,
-                                            absl::string_view address,
+                                            absl::string_view name,
                                             absl::string_view default_port,
                                             absl::Time deadline) = 0;
     /// Asynchronously perform an SRV record lookup.
@@ -331,8 +337,10 @@ class EventEngine {
   // de-experimentalize this API.
   virtual bool IsWorkerThread() = 0;
 
-  /// Creates and returns an instance of a DNSResolver.
-  virtual std::unique_ptr<DNSResolver> GetDNSResolver() = 0;
+  /// Creates and returns an instance of a DNSResolver, optionally configured by
+  /// the \a options struct.
+  virtual std::unique_ptr<DNSResolver> GetDNSResolver(
+      const DNSResolver::ResolverOptions& options) = 0;
 
   /// Asynchronously executes a task as soon as possible.
   ///