Firestore project: Import of API interfaces.

Import of firebase-ios-sdk from Github.

  - 32266c5 Make sure Firestore/core/include is in the podspec (firebase#748) by Gil <mcg>
  - 070c0ea Merge pull request firebase#746 from morganchen12/unguarded-block by Morgan Chen <[email protected]>
  - 13f572e Increase expectation timeout to 25 seconds. (firebase#744) by Michael Lehenbauer <[email protected]>
  - 47d81fd fix (firebase#739) by Chen Liang <chliang>
  - 515625c Remove predecessorKey,Object,Document, etc (firebase#735) by Gil <mcg>
  - ac30522 Start on ArraySortedMap in C++ (firebase#721) by Gil <mcg>
  - 729b8d1 Move all Firestore Objective-C to Objective-C++ (firebase#734) by Gil <mcg>
  - 693d064 Merge pull request firebase#732 from OleksiyA/FCM-subscribe-compl... by Oleksiy Ivanov <[email protected]>
  - 3cbdbf2 Schema migrations for LevelDB (firebase#728) by Greg Soltis <gsoltis>
  - 6474a82 Firestore DatabaseId in C++ (firebase#727) by zxu <zxu>
  - 05ef5ae Add absl_strings to firebase_firestore_util_test dependen... by rsgowman <rgowman>
  - 03a0069 Add changelog entry for my last PR (oops) and also add a ... by Michael Lehenbauer <[email protected]>
  - 5a280ca  Import iterator_adaptors from google3 (firebase#718) by Gil <mcg>
  - 63a380b Use fixed-sized types (firebase#719) by Gil <mcg>
  - 6dc1373 Version updates to 4.8.2 (firebase#722) by Paul Beusterien <paulbeusterien>
  - a74d39f Fix a number of c++ build errors (firebase#715) by rsgowman <rgowman>
  - cceeec3 travis: check for copyright in sources (firebase#717) by Paul Beusterien <paulbeusterien>
  - 4d7c973 Fix b/72502745: OnlineState changes cause limbo document ... by Michael Lehenbauer <[email protected]>
  - af6976a normalize and port the rest of Firebase/Port code (firebase#713) by zxu <zxu>
  - 7226b4a port TargetIdGenerator to iOS (firebase#709) by zxu <zxu>
  - 5306474 adding unit test for auto init enable function (firebase#710) by Chen Liang <chliang>
  - 821fb90 implement `TargetIdGenerator` in C++ for Firestore (firebase#701) by zxu <zxu>
  - 5fdda3f normalize string_util (firebase#708) by zxu <zxu>
  - 15a2926 Update CHANGELOG for M21.1 release (firebase#679) by Ryan Wilson <wilsonryan>
  - f027214 Fix incorrect deprecation message (firebase#688) by Iulian Onofrei <[email protected]>
  - bfa0e40 Implement the rest of FieldValue types for C++ (firebase#687) by zxu <zxu>
  - 1f77212 Properly publish Abseil sources as a part of the podspec ... by Gil <mcg>

ORIGINAL_AUTHOR=Gil <[email protected]>
PiperOrigin-RevId: 184568931