Refactor BugsnagApiClient

[nickdowell]

Goal

Make a change required for efficient implementation of breadcrumb trimming functionality (coming in a subsequent PR).

Changeset

Changes BugsnagApiClient to accept NSData and leave JSON encoding to callers - this is the change required for efficient trimming.

Makes BugsnagApiClient responsible for setting Bugsnag-Sent-At header.

Removes unused BugsnagConfiguration.sessionApiHeaders.

Replaces BugsnagApiClient class with equivalent C functions.

Renames BugsnagApiClientDeliveryStatus to BSGDeliveryStatus.

Simplified integrity header generation by including "sha1 " prefix in function's result.

Testing

Covered by existing unit and E2E tests.

[github-actions]

Bugsnag.framework binary size decreased by 3,088 bytes from 774,592 to 771,504 🎉

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0   +26% +3.02Ki    [__LINKEDIT]
   +13% +1.74Ki   +13% +1.74Ki    [__TEXT]
  +7.7%    +836   +72%    +836    [__DATA]
  +0.3%      +8  +0.3%      +8    Binding Info
  -2.3%      -8  -2.3%      -8    __DATA,__objc_classlist
  -1.2%     -12  -1.2%     -12    __DATA,__objc_ivar
  -0.9%     -16  -0.9%     -16    Function Start Addresses
  -1.8%     -16  -1.8%     -16    [2 Others]
  -0.5%     -16  -0.5%     -16    __TEXT,__unwind_info
  -1.7%     -19  -1.7%     -19    __TEXT,__objc_classname
  -1.7%     -32  -1.7%     -32    Rebase Info
  -0.2%     -32  -0.2%     -32    __DATA,__cfstring
  -0.6%     -40  -0.6%     -40    __DATA,__objc_selrefs
  -2.4%     -67  -2.4%     -67    __TEXT,__objc_methtype
  -2.3%     -80  -2.3%     -80    __DATA,__objc_data
  -0.5%     -94  -0.5%     -94    __TEXT,__cstring
  -1.0%    -266  -1.0%    -266    __TEXT,__objc_methname
  -1.3%    -648  -1.3%    -648    __DATA,__objc_const
  -0.9% -1.20Ki  -0.9% -1.20Ki    Symbol Table
  -0.5% -1.29Ki  -0.5% -1.29Ki    __TEXT,__text
  -1.0% -1.77Ki  -1.0% -1.77Ki    String Table
  -0.4% -3.02Ki  [ = ]       0    TOTAL

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[kattrali]

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