fix/typings: Remove Readonly<T> from AuthenticateHandler params

[bdchauvette]

This commit removes the Readonly wrappings around the username and password params to the AuthenticateHandler interface, which were added in #596.

The motivation for this change is that when password is Readonly<Buffer>, the type is incompatible with crypto.timingSafeEqual, which is the function Aedes users should be using to compare raw, sensitive buffers with each other.
Because Readonly<Buffer> is incompatible with crypto.timingSafeEqual, users end up having to cast with password as Buffer, which largely defeats the purpose of marking it Readonly in the first place and introduces casting in security-related areas of the code where it's not really needed in the first place.

The error it gives is:

No overload matches this call.
  Overload 1 of 2, '(a: ArrayBufferView, b: ArrayBufferView): boolean', gave the following error.
    Argument of type 'Readonly<Buffer>' is not assignable to parameter of type 'ArrayBufferView'.
      Type 'Readonly<Buffer>' is missing the following properties from type 'Float32Array': [Symbol.iterator], [Symbol.toStringTag]
  Overload 2 of 2, '(a: ArrayBufferView, b: ArrayBufferView): boolean', gave the following error.
    Argument of type 'Readonly<Buffer>' is not assignable to parameter of type 'ArrayBufferView'.ts(2769)

Removing it from username has no effect, because strings are already immutable in JavaScript, and TypeScript will automatically treat it as if it were just string.

[robertsLando]

LGTM

[coveralls]

Pull Request Test Coverage Report for Build 793693562

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.911%

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Totals
Change from base Build 774409615:	0.0%
Covered Lines:	800
Relevant Lines:	800

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💛 - Coveralls

[getlarge]

Even though i understand the intent of marking them as Readonly, i also had the same typing problem.

[getlarge]

I think @gnought introduced this type change few weeks ago, it would be nice to have his opinion before merging.

[robertsLando]

@mcollina Thoughts?