Support bearer_token_file option on HTTP output plugin

[rakelkar]

Describe the solution you'd like
Would be great if the HTTP plugin supported reading a file to retrieve the token to use for the HTTP auth header. This is similar to the bearer_token_file option on Prometheus remote write config.

Describe alternatives you've considered
The plugin does support headers as part of its configuration, however as noted in #365 since dynamic config reload is not supported, workarounds are required to refresh the token.

[ecusnir]

The code includes bearer_token_file parameter.

Unfortunately, it is only read once, unlike prometheus that uses a round tripper to update the token.

[github-actions]

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[github-actions]

This issue was closed because it has been stalled for 5 days with no activity.