Added changes for self signed cert generation

[kang-mansher]

Description

Added new SelfSignedCertGenerator to generate a self-signed certificate using SUN security provider in JVM
Bouncycastle provider(which was causing Runtime error - Noclassdeffound) was fetched from linecorp armeria library, which also has some dependencies in other parts. So I could not exclude transient dependency from the armeria.

Find attached the screenshot of the error in the existing fat jar and a successful server startup after the fix.

Error:

After Fix:

• Relevant Issues: Mock fails to launch when key & cert have to be auto-generated #2270

• Type of change :

  • Bug fix for existing feature

[staffier]

Sorry for the delay -- looks great!

% java -jar karate-1.5.0-SNAPSHOT.jar -m ~/karate/karate-core/src/test/java/com/intuit/karate/fatjar/server.feature -s
17:24:37.922 [main] INFO com.intuit.karate - Karate version: 1.5.0-SNAPSHOT
17:24:38.287 [main] WARN c.i.karate.http.SslContextFactory - ssl - cert.pem and / or key.pem not found, will create
17:24:39.500 [main] INFO com.intuit.karate - mock server initialized: ../src/test/java/com/intuit/karate/fatjar/server.feature
17:24:40.441 [main] DEBUG com.intuit.karate.http.HttpServer - server started: ml40j6md6r:8080