feat: add getKeys method and test

[jacksteamdev]

Community guidelines:

• I have read and understand the
  contributing guidelines
  and
  code of conduct

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers: #15

Description

It's useful to be able to retrieve the keys that a storage bucket is using.

Usage

const keys = await bucket.getKeys()

console.log(keys) // array of key names that are stored in the bucket