feat: update getSecrets types

[dwwoelfel]

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Which problem is this pull request solving?

Contentful and sanity are missing in autocomplete in the return value of getSecrets

Describe the solution you've chosen

Adds sanity and contentful to the type on NetlifySecrets

Also gets the fieldName from the graphql query so that we don't have to normalize the field names or camelCase them ourselves.

Checklist

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