fix(issue-640): adding headers in task request samples

[pattishin]

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Fixes #640 🦕

Affects:

• https://cloud.google.com/tasks/docs/creating-http-target-tasks
• https://cloud.google.com/run/docs/triggering/using-tasks

[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml