…ertificateExtension (#6519)Motivation:This is the first of a series of PRs included by#6516.Currently, our mTLS tests are not realistic in that:- Certificate chains aren't verified- We set TLS configurations to not verify the peer, which means inreality there's no point in using mTLS.For this, I propose that a `SignedCertificate` is introduced. This is acertificate which is signed by another certificate (another`SignedCertificate` or `SelfSignedCertificate`). A JUnit `Extension` hasalso been introduced so users can easily test this behavior.In the process, I found that `SelfSingedCertificateNameType` has a typo,and isn't adding much value. This enum has been removed.Modifications:- Introduced `SignedCertificate`, `SignedCertificateExtension`- `SelfSignedCertificate`, `SelfSignedCertificateExtension` inherits thenewly introduced constructs- Default behavior has been modified so that all generated certificatescan act as a ca.- `SelfSingedCertificateNameType` has been removedResult:- Users can easily create a certificate chain for testing using`SignedCertificateExtension`<!--Visit this URL to learn more about how to write a pull requestdescription:https://armeria.dev/community/developer-guide#how-to-write-pull-request-description-->