Adding appendHex method to StringBuilder

[温绍锦(高铁)]

There are many combinations of `append(Integer.toHexString(i))` and `append(Long.toHexString(i))` in the JDK code. I request to add method `appendHex(int)` and `appendHex(int)` to StringBuilder and StringBuffer. This can reduce duplicate code within the JDK and will also be useful to users.
I submitted a PR ( https://github.com/openjdk/jdk/pull/15998 <https://github.com/openjdk/jdk/pull/15998 > ), including the code and tests for the new method. I also replaced the code that uses the append + toHexString combination inside the JDK with appendHex.
Please review and don't hesitate to critique my approach and patch.
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