Update Spring Boot nested JarFile support to improve the performance of
signed jars. Prior to this commit, `certificates` and `codeSigners`
were read by streaming the entire jar whenever the existing values
were `null`. Unfortunately, the contract for `getCertificates` and
get `getCodeSigners` states that `null` is a valid return value. This
meant that full jar streaming would occur whenever either method was
called on an entry that had no result. The problem was further
exacerbated by the fact that entries might not be cached.

See gh-19041