fix: typings of fastify.cache.get

[Uzlopak]

Resolves #132

Question is, if a non callback .get() is returning a promise.
I also assumed that if the key has no value it returns undefined.

Before merging it would be good to clear this.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Uzlopak]

Checked...
If key has no value fastify.cache.get will return null
If useAwait for abstractcache is set, we need to return a promise
The return value if existing is an object with the keys: item, ttl and stored.

[jsumners]

Is there a specific question you need my input on?

[Uzlopak]

No I tested it and set the types based on the return values
Also sry, i forgot you dont want to review typescript related PRs.

[gurgunday]

looks good to me