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GH-39014: [Java] Add default truststore along with KeychainStore when on Mac system (#39235)

### Rationale for this change
As described in #39014, when using the system TrustStore on Mac, the certificates returned do not include Root CAs trusted by the system. This change adds the default KeyStore instance along with the KeyChainStore to include trusted Root CAs. The reason we add the default KeyStore instance is because there is no easy way to get the certificates from the System Roots keychain. 

### What changes are included in this PR?

I've updated ClientAuthenticationUtils to get the default KeyStore instance when the operating system is macOS and have updated the tests to include this change. 

### Are these changes tested?

See changes made in ClientAuthenticationUtilsTest.java.

### Are there any user-facing changes?

No

* Closes: #39014

Authored-by: Ravjot Brar <ravjot.brar@dremio.com>
Signed-off-by: David Li <li.davidm96@gmail.com>