feat: add serverless-domain-manager config helpers

[dustinsgoodman]

Closes #106

• Add serverless-domain-manager
• Add helper for configuring the settings
• Show where to add the plugin in public-api

Current Debates:

• Should this even be included in this template?
• Is the override too restrictive? See docs for all different config options: https://github.com/amplify-education/serverless-domain-manager#installing
• Does it need better documentation or examples? For instance, each service could theoretically be deployed to a subdomain but that defeats the purposes of the pattern of a single service for public interaction.

[codecov]

Codecov Report

Merging #248 (397b5d8) into main (6414e86) will not change coverage.
The diff coverage is n/a.

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##              main      #248   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           18        18           
  Lines          117       117           
  Branches        11        11           
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  Hits           117       117           

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