Support PluginConfigVariable for Confluence/Jira bearer token to enable secrets manager integration #6844

Description

The Confluence and Jira Data Center sources currently accept bearer_token as a plain String in AuthenticationConfig. This works for static Personal Access Tokens (PATs), but does not support reading credentials from external secrets managers (e.g., AWS Secrets Manager).

Per discussion in #6769 with @dlvenable, the bearer token field should be changed from String to PluginConfigVariable to enable:

1. External secret refresh : somebody outside of Data Prepper updates the secret in AWS Secrets Manager, and Data Prepper picks up the new value on next use.

This is the same pattern used by RDS source, OpenSearch sink, and other plugins that read credentials externally.

Context

• Parent issue: Confluence Data Center #6496 (Confluence Data Center support)
• Implementation PR: Add Confluence Data Center support with allow_local_address and beare… #6769
• Reviewer comment:
  https://github.com/opensearch-project/dataprepper/pull/6769#discussion_r3196227674

References

• PluginConfigVariable usage in OpenSearch sink: ConnectionConfiguration.java
• PluginConfigVariable usage in RDS source: RdsSourceConfig.java