Restore CharSequences.newAsciiString(String) (#1503)

Motivation:
In ServiceTalk 0.39.0 the method
`CharSequences.newAsciiString(CharSequence)` was introduced which
replaced `CharSequences.newAsciiString(String)`. This was a source
compatible, but binary incompatible change. All existing uses of the
`String` overload could be recompiled to use the `CharSequence`
overload. Existing binary code expected the `String` overload to exist
and fails with `NoSuchMethodError` at runtime.
Modifications:
For cases where the libraries and applications using ServiceTalk cannot
be recompiled conveniently the `CharSequences.newAsciiString(String)`
overload is restored.
Result:
Binary compatibility with libraries and applications which use
`CharSequences.newAsciiString(String)` and are compiled against
versions of ServiceTalk before 0.39.0 is restored.

servicetalk-buffer-api/src/main/java/io/servicetalk/buffer/api/CharSequences.java

@@ -55,6 +55,17 @@ private CharSequences() {
         // No instances
     }
 
+    /**
+     * Create a new {@link CharSequence} from the specified {@code input}, supporting only 8-bit ASCII characters, and
+     * with a case-insensitive {@code hashCode}.
+     *
+     * @param input a character sequence containing only 8-bit ASCII characters.
+     * @return a {@link CharSequence}
+     */
+    public static CharSequence newAsciiString(final String input) {
+        return newAsciiString((CharSequence) input);
+    }
+
     /**
      * Create a new {@link CharSequence} from the specified {@code input}, supporting only 8-bit ASCII characters, and
      * with a case-insensitive {@code hashCode}.