RFR: 8334772: Change Class::signers to an explicit field

[Chen Liang]

`Class` has 2 VM-injected fields that can be made explicit: `Object[] signers` and `ProtectionDomain protectionDomain`. We make the signers field explicit. (The ProtectionDomain can be revisited when SecurityManager is removed, as SecurityManager is accessing it via JNI as well.)

Migrate the JNI code to Java. The getter previously had a redundant primitive type check, which is dropped in the migrated Java code. The `Object[] getSigners` is no longer `native`, thus requiring a CSR record. Reviewers please help review the associated CSR.

-------------

Commit messages:
 - 8334772: Change Class::signers to an explicit field

Changes: https://git.openjdk.org/jdk/pull/20223/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20223&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334772
  Stats: 71 lines in 6 files changed: 7 ins; 53 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/20223.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20223/head:pull/20223

PR: https://git.openjdk.org/jdk/pull/20223