Encrypt stored client secret value

[zbynek]

Fixes https://issues.jenkins.io/browse/SECURITY-1891 from security advisory

When serializing, the encrypted value is used. Deserialization checks if the string is in format {encrypted} and treats it as encrypted or plain text accordingly.

This includes changes from #58 so that the CI build can be downloaded and used for fixing both vulnerabilities.

CC @jvz @Wadeck

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[Kevin-CB]

I manually tested this fix for SECURITY-1833 & SECURITY-1891 and it's look complete.

However, you should changed in GitLabSecurityRealm/config.jelly the <f:textbox /> by a <f:password /> since it's a secret.

Recommendation:
https://www.jenkins.io/doc/developer/security/secrets/#secrets-and-configuration-forms

[zbynek]

@Kevin-CB thanks for reviewing both issues. I've changed the control to password as you suggested. Btw. I don't have access to either of the Jira issues, so if anything is needed from me to get these PRs merged, please let me know here.

[basil]

Thanks for the PR!

[basil]

Released in 1.14.