fix(firestore): fix firestore settings type definition

[kmsbernard]

Description

Fix firestore.Settings type definition to make each field could be optional. As per implementation of firebase.firestore().settings(), it was intended to be optional.

And also follows firebase-js-sdk's type definitions: https://github.com/firebase/firebase-js-sdk/blob/master/packages/firestore-types/index.d.ts

Related issues

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

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[codecov]

Codecov Report

Merging #4965 (874c61b) into master (bc893ab) will decrease coverage by 52.37%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #4965       +/-   ##
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- Coverage   89.21%   36.84%   -52.36%     
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  Files         105       48       -57     
  Lines        3482     1455     -2027     
  Branches        0      348      +348     
===========================================
- Hits         3106      536     -2570     
- Misses        376      697      +321     
- Partials        0      222      +222     

[mikehardy]

Indeed! Thanks for the link to the firebase-js-sdk for quick verification
You are correct and this looks good to me