Added optional to args that get and set

[Dahaden]

Description

Noticed some APIs have get and set functionality bu the typescript types didnt reflect this.

Soooo happy to see updated being made to this repo and especially the adoption of typescript!! Thank you so much!

Test plan

• Testing completed successfully by @juliofarah using internal tools and e2e tests:
  
  

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Release plan

New version is not required because this is a dev-only change

[Dahaden]

Also, If you have any time, this is something that we have found causes a few issues with us on some clients :(
segmentio/localstorage-retry#17

[juliofarah]

@Dahaden thank you so much for opening this PR! I'm so happy to see this. One of the biggest reasons why started the Typescript migration was to enable contributions like this!

I'm going to spend a few minutes testing your changes end to end with a few other internal services we have and, if everything looks fine, I'll merge it right away.

Thanks again for your contribution

[juliofarah]

Great job, @Dahaden
I have pulled this branch locally and tested against some internal tools - and took the liberty of updating the PR description with screenshots taken from those tests.

We really appreciate your contribution, looking forward to seeing more of these 🏅