Convert all relevant `string` params to `env_var_name` about circleci-orbs HOT 3 CLOSED

@etorix @KyleTryon no pressure but if ya'll wanna help w/ this, you're more than welcome to !

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@iynere Any thoughts on situations where we do not want to require an environment variables but leave it as an option where the default could be an environment variable but can be overwritten by a string if needed?

This is currently how the webhook is set for the Slack Orb. Currently, an env var is the default but I want to leave the ability to add a custom webhook url at any point.

example: https://github.com/CircleCI-Public/slack-orb/blob/master/src/%40orb.yml#L16

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@KyleTryon in that case, i'd say leave it a string 😄

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