feat: Vercel Metadata Override

[cgosiak]

This PR adds the ability to optionally override vercel metadata flags.

Presently, if a metadata tag is supplied to the custom action (githubCommitSha for example); it will be overwritten by what is supplied via the actions/core library. I took a minimally invasive approach here by allowing for backwards compatibility without the need for adding additional params to the action call.

[github-actions]

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[github-actions]

Deploy preview for express-basic-auth ready!

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[github-actions]

Deploy preview for zeit-now-deployment-action-example-static ready!

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https://staging.static.vercel-action.amond.dev
https://pr-118.static.vercel-action.amond.dev

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[github-actions]

Deploy preview for zeit-now-deployment-action-example-angular ready!

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https://pr-118.angular.vercel-action.amond.dev

Built with commit 459dc97.
This pull request is being automatically deployed with vercel-action

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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