[PLAT-8726] Use device.id as fallback user.id for events & sessions

[nickdowell]

Goal

Makes behaviour of user.id consistent with other platform, crash errors, and our online documentation.

After calling Bugsnag.setUser(nil, withEmail: nil, andName: nil), events and sessions should have their user.id set to the default Bugsnag-generated device ID.

Changeset

Adds -[BugsnagUser withUserId] to get a user object with the fallback id if required.

Updates BugsnagSessionTracker and BugsnagClient to use this method to populate sessions and events with a user.id.

Renames -[BugsnagUser initWithUserId:name:emailAddress:] to -[BugsnagUser initWithId:name:emailAddress:].

Removes unused method +[BugsnagUser userWithUserId:name:emailAddress:].

Simplifies -[BugsnagConfiguration getPersistedUserData].

Testing

Amends existing and adds a new scenario to verify that fallback user.id is sent when expected, while retaining ability to nullify it via callbacks.

[github-actions]

Bugsnag.framework binary size decreased by 168 bytes from 805,032 to 804,864 🎉

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +216  +0.1%    +216    __TEXT,__text
  [ = ]       0  +1.2%    +168    [__LINKEDIT]
  +0.1%      +8  +0.1%      +8    __DATA,__objc_selrefs
  +0.1%      +4  +0.1%      +4    __TEXT,__unwind_info
  -0.0%      -8  -0.0%      -8    __DATA,__objc_const
  -0.1%     -31  -0.1%     -31    __TEXT,__objc_methname
  -0.0%     -32  -0.0%     -32    Symbol Table
  -0.1%    -136  -0.1%    -136    String Table
  -0.7%    -189  -0.7%    -189    [__TEXT]
  -0.0%    -168  [ = ]       0    TOTAL

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