OpenTelemetry input - Allow specifying TLS cert paths instead of requiring upload

## What?

Customer request: https://github.com/Graylog2/support/issues/278

Some inputs allow specifying a TLS Client Auth Trusted Certs File or Directory that reside on the Graylog server.
The OpenTelemetry input requires users to upload the TLS certs.

## Why?

Configuration of certificates should be the same for all types of inputs, to make administration easier and increase user acceptance.

## Your Environment

* Graylog Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: