feat: Add metadata accessors to BugsnagEvent

[robinmacharg]

Also added a BaseUnitTest to collect common setup patterns.

Goal

Add metadata accessor methods to BugsnagEvent, to align it with other metadata-utilising classes.

Design

Implement the necessary methods (add*, clear* and get*), add tests. These methods deal with NSDictionarys, not BugsnagMetadata

Changeset

Changes primarily to BugsnagEvent.

Tests

Added tests that the new methods performed as expected.

Also created a top-level Unit Test base class for common setup tasks. There's a question over whether it would be clearer to have a helper class with static methods.

Review

Outstanding Questions

See above.

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

• The correct target branch has been selected (master for fixes, next for
  features)
• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[kattrali]

The implementation looks good. There might be a couple of missing tests in the middle?