Add first party Objective-C to Swift SDK RPC migration shim.

[julianlocke]

See dropbox/SwiftyDropbox#435 for both output code from these changes and a discussion of them. This code to be deleted post migration.

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 265 lines in your changes missing coverage. Please review.

Project coverage is 48.45%. Comparing base (40bd164) to head (bd7e81e).

Files with missing lines	Patch %	Lines
stone/backends/swift.py	0.00%	163 Missing ⚠️
stone/backends/swift_types.py	0.00%	52 Missing ⚠️
stone/backends/swift_client.py	0.00%	38 Missing ⚠️
stone/backends/swift_helpers.py	0.00%	12 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           objc-shim     #350      +/-   ##
=============================================
- Coverage      49.85%   48.45%   -1.40%     
=============================================
  Files             37       37              
  Lines           8778     9031     +253     
  Branches        1891     1963      +72     
=============================================
  Hits            4376     4376              
- Misses          4084     4337     +253     
  Partials         318      318              

Flag	Coverage Δ
unit	48.45% <0.00%> (-1.40%)	⬇️

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