Config via URL, security enhancement

### Use Case

It's great to pull the config from an URL. Problem is, that this is a command line parameter which is visible to anyone on the system.
It would be great to only have a minimal config file locally, which tells Telegraf where to look for the actual config. We could also configure things like: disable TLS verification and the config-url-watch-interval in this local config

### Expected behavior

It would be great to use a local config file to tell Telegraf where to look for the actual config. This would improve the security, as we use the config roll-out via an URL with an API-key in it. We plan to also send config information which may contain sensitive data like login to a database to check if it's still alive.
The local file could be put to a place where only admins have access to.

### Actual behavior

Currently anyone on the local system can see the command line and copy the URL and is able to see the config data sent by the server.

### Additional info

_No response_

Start by tracing how the command-line config URL is handled and how the local Telegraf configuration is parsed. Define how a local file should provide the remote URL, TLS verification setting, and config-url-watch-interval. Done means the URL and related settings can be supplied through the protected local file without exposing the API key in the process command line.

Written by the indexing model from the issue text.