Feature request: config parameter for configuration style legacy / yaml regardless of file extension.

**Is your feature request related to a problem? Please describe.**

We want to use Graylog-Sidecar for rollout configuration and process management. But Graylog-Sidecar supports **only** `.conf` as extension. Starting fluent-bit ends up in a failure, because content is YAML, but extension is .conf and fluent-bit expects legacy content.

https://github.com/Graylog2/collector-sidecar/issues/513

**Describe the solution you'd like**
It would be good to have a configuration parameter, to let fluent-bit know, which configuration style it should use, independent of the file extension.

```
--config-style=yaml --config ./my-config.conf # with YAML config
--config-style=classic # with legacy config
```
**Describe alternatives you've considered**
I've asked Graylog-sidecar, if there is a workaround, but there is nothing. What in theory works: Use a symlink from .conf to .yaml, so both are happy, but it does not work this way, like Graylog-sidecar handle this deployment. It generates the .conf file in a temp created (uuid?) folder and starts the process (fluent-bit). Which means fluent-bit gets the .conf file.

**Additional context**
We need to replace Nxlog anywhere, which gets its configuration file also via Graylog. Every service or VM has its custom / role based config and we want fluent-bit as replacement for it.

Start at fluent-bit's --config handling and trace how the configuration file extension selects legacy versus YAML parsing. Use the requested --config-style=yaml and --config-style=classic examples to define the behavior, then verify that each style works independently of the file extension and that existing configuration behavior remains unchanged.

Written by the indexing model from the issue text.