feat: Add SecureEncoder support to PFFileObject

[dhoerl]

New Pull Request Checklist

• [X ] I am not disclosing a vulnerability.
• [X ] I am creating this PR in reference to an issue.

Issue Description

PFFileObjects can be used outside of a PFObject subclass. When archiving them and other objects using the NSSecureEncoder, the encoder now fails with an error that PFFileObjects are not secure coding compliant.

Fine - in Objective-C, I create a class extension to add it. Works fine. However, when I go to change that extension to Swift, the compilation fails with the error that NSSecureCoding cannot be added in a class extension (but must be done by the original class).

Closes: #1736

Approach

Add NSSecureCoding to PFFileObject:

• return YES from the class method supportsSecureCoding
• encode the object using the existing PFEncoder objectEncoder
• decode the object using PFDecoder objectDecoder

TODOs before merging

• Add tests
• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[parse-github-assistant]

I will reformat the title to use the proper commit message syntax.

[mtrezza]

Would you want to add a test so we can merge this?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 3.77%. Comparing base (dd05d41) to head (0d28a0f).
Report is 3 commits behind head on master.

❗ Current head 0d28a0f differs from pull request most recent head ea4346d. Consider uploading reports for the commit ea4346d to get more accurate results

Files	Patch %	Lines
Parse/Parse/Source/PFFileObject.m	0.00%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #1768       +/-   ##
==========================================
- Coverage   64.24%   3.77%   -60.48%     
==========================================
  Files         201     155       -46     
  Lines       23233   17952     -5281     
==========================================
- Hits        14926     677    -14249     
- Misses       8307   17275     +8968     

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