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[bot]]

Thanks for opening this pull request!

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

[parse-github-assistant[bot]]

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[bot]]

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.

:exclamation: 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 :warning:

Additional details and impacted files

```diff @@ 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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