string: add AsciiCaseInsensitive{KeyEqual, Hash} for unordered map

https://httpwg.org/specs/rfc9110.html#rfc.section.5.1
Field names in HTTP headers are case-insensitive. These
structs will be used in the headers map to search by key.

In libevent field names are also converted to lowercase for comparison:
  evhttp_find_header()
  evutil_ascii_strcasecmp()
  EVUTIL_TOLOWER_()

src/util/strencodings.h

@@ -13,6 +13,7 @@
 #include <span.h>
 #include <util/string.h>
 
+#include <algorithm>
 #include <array>
 #include <bit>
 #include <charconv>
@@ -353,6 +354,20 @@ struct Hex {
 };
 } // namespace detail
 
+struct AsciiCaseInsensitiveKeyEqual {
+    bool operator()(std::string_view s1, std::string_view s2) const
+    {
+        return ToLower(s1) == ToLower(s2);
+    }
+};
+
+struct AsciiCaseInsensitiveHash {
+    size_t operator()(std::string_view s) const
+    {
+        return std::hash<std::string>{}(ToLower(s));
+    }
+};
+
 /**
  * ""_hex is a compile-time user-defined literal returning a
  * `std::array<std::byte>`, equivalent to ParseHex(). Variants provided: