Feature Request: OAuth2 private key jwt client assertion support

Feature Request: OAuth2 private key jwt client assertion support: a task in LegoFlow-SWE (Harbor dataset). Current behavior Fluent Bit’s OAuth2 plugin only supports client secret -based authentication. This makes it impossible to integrate with identity providers (such as ADFS) that require RFC…

The task

**Current behavior** Fluent Bit’s OAuth2 plugin only supports `client_secret`-based authentication. This makes it impossible to integrate with identity providers (such as ADFS) that require RFC 7523 private_key_jwt client assertions.

Part of Lego-X/LegoFlow-SWE.