Narrow Resolv::DNS resource methods by typeclass via bounded generic

`Resolv::DNS#getresource` / `#getresources` / `#each_resource` /
`#extract_resources` accept a typeclass argument (e.g.
`Resolv::DNS::Resource::IN::MX`) that fully determines the returned
resource subclass. The signatures returned the upper bound
`Resolv::DNS::Resource`, dropping that information; callers using
subclass-specific methods like `MX#exchange` then need a downcast.

Use a bounded type parameter (`[T < Resolv::DNS::Resource]`) so the
return / block parameter tracks the typeclass:

    def getresources: [T < Resolv::DNS::Resource] (dns_name, singleton(T)) -> Array[T]
                    | (dns_name, singleton(Resolv::DNS::Resource::ANY)) -> Array[Resolv::DNS::Resource]

`Resolv::DNS::Resource::ANY` does not inherit from `Resource` (it sits
parallel under `Query`), so the wide overload is kept alongside as the
fallback for `ANY` queries.

`fetch_resource` also takes a typeclass argument but does not return or
yield a `Resource`, so it is left as-is.

Author: zonuexe

stdlib/resolv/0/resolv.rbs

@@ -218,9 +218,11 @@ class Resolv::DNS
   # Iterates over all `typeclass` DNS resources for `name`.  See #getresource for
   # argument details.
   #
-  def each_resource: (dns_name name, singleton(Resolv::DNS::Query) typeclass) { (Resolv::DNS::Resource) -> void } -> void
+  def each_resource: [T < Resolv::DNS::Resource] (dns_name name, singleton(T) typeclass) { (T) -> void } -> void
+                   | (dns_name name, singleton(Resolv::DNS::Resource::ANY) typeclass) { (Resolv::DNS::Resource) -> void } -> void
 
-  def extract_resources: (Resolv::DNS::Message msg, dns_name name, singleton(Resolv::DNS::Query) typeclass) { (Resolv::DNS::Resource) -> void } -> void
+  def extract_resources: [T < Resolv::DNS::Resource] (Resolv::DNS::Message msg, dns_name name, singleton(T) typeclass) { (T) -> void } -> void
+                       | (Resolv::DNS::Message msg, dns_name name, singleton(Resolv::DNS::Resource::ANY) typeclass) { (Resolv::DNS::Resource) -> void } -> void
 
   # <!--
   #   rdoc-file=lib/resolv.rb
@@ -299,7 +301,8 @@ class Resolv::DNS
   # Returned resource is represented as a Resolv::DNS::Resource instance, i.e.
   # Resolv::DNS::Resource::IN::A.
   #
-  def getresource: (dns_name name, singleton(Resolv::DNS::Query) typeclass) -> Resolv::DNS::Resource
+  def getresource: [T < Resolv::DNS::Resource] (dns_name name, singleton(T) typeclass) -> T
+                 | (dns_name name, singleton(Resolv::DNS::Resource::ANY) typeclass) -> Resolv::DNS::Resource
 
   # <!--
   #   rdoc-file=lib/resolv.rb
@@ -308,7 +311,8 @@ class Resolv::DNS
   # Looks up all `typeclass` DNS resources for `name`.  See #getresource for
   # argument details.
   #
-  def getresources: (dns_name name, singleton(Resolv::DNS::Query) typeclass) -> Array[Resolv::DNS::Resource]
+  def getresources: [T < Resolv::DNS::Resource] (dns_name name, singleton(T) typeclass) -> Array[T]
+                  | (dns_name name, singleton(Resolv::DNS::Resource::ANY) typeclass) -> Array[Resolv::DNS::Resource]
 
   def lazy_initialize: () -> untyped