chore: collapse multiple mail-send TS signatures

[seromenho]

From typescript docs overloads should be used if

...will return two different things based on what the user has passed in

Here is not the case, so we can have one signature only where the first argument can be one thing or the other.

With this we can have a wrapper for the send that will retain all the types and allow both types on the first param.

eg:

const wrap = <T extends Array<any>, U>(fn: (...args: T) => U) => {
  return (...args: T): U => fn(...args)
}

const send = wrap(sgMail.send)

Before this PR and because both signatures had the same number of params I was having troubles (if it's possible) to retain both signatures. It was picking the last one.

Checklist

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• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified