[extension/bearertokenauthextension] support reading tokens from file

[frzifus]

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

As of now, a bearer token must be provided in the extension configuration. We would like to make bearer tokens available via a k8s secret.

(token will be refreshed from time to time)

Describe the solution you'd like

Extract the token from a file. At best not with every request, but only if the content of the file was changed.

Describe alternatives you've considered

Create another extension.

Additional context

No response

[frzifus]

Created a proposal as pr.

[github-actions]

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[frzifus]

Supported since #14326