added login page

[johnneal3]

I have a requirement on my end to secure access to Kafdrop via a login page. I am submitting this code for your review / approval. This is a simple login solution, only supporting a single user.

Default:

• username = admin
• password = admin

Override via command line args (validated in docker/docker-compose as well):

java --add-opens=java.base/sun.nio.ch=ALL-UNNAMED    \
     -jar target/kafdrop-3.28.0-SNAPSHOT.jar \
      --kafka.brokerConnect=localhost:29092    \
      --spring.security.user.name=fred --spring.security.user.password=mary

[lgtm-com]

This pull request introduces 1 alert when merging bed6777 into fdc887a - view on LGTM.com

new alerts:

• 1 for Disabled Spring CSRF protection

[xtruan]

I have tested this enhancement (verifying login prompt, creating topics, deleting topics, etc.) at my organization and we have incorporated into our Kafdrop instance. Thanks!