Implement Client key file and Client certificate file for all otlp exporters

[lzchen]

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#configuration-options

Include the environment variables as well.

[Emmaka9]

Is it resolved? If not, I would like to work on it.

[srikanthccv]

@Emmaka9 are you working on this? Let us know if you need any help.

[chittalpatel]

Hi @srikanthccv @lzchen, I think I can implement this. I'll look into this.

[sandy2008]

Hi, based on @chittalpatel 's PR, we made the fix in https://github.com/open-telemetry/opentelemetry-python/pull/4116

[emdneto]

Closing as fixed by https://github.com/open-telemetry/opentelemetry-python/pull/4116