online_image: allow setting templatable headers on http request

[afflux]

Description:

The HTTP requests by the online_image component did not give the user a config option to set the HTTP headers. PR below implements this.

Pull request in esphome with YAML changes (if applicable): esphome/esphome#7767

Checklist:

• [x] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• [ ] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [ ] Link added in /index.rst when creating new documents for new components or cookbook.

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