Add bugsnag.clearMetadata()

[robinmacharg]

Goal

To implement Bugsnag.clearMetadata() variants.

Design

This has been implemented for both section and key variants in the top-level Bugsnag class as well as the backing BugsnagMetadata class. Deviates mildly from the spec. in method message naming. Swift methods have been exposed appropriately on Bugsnag only.

Implemented as simple dictionary operations with existence checks if appropriate.

Changeset

Bugsnag and BugsnagMetadata classes andd associated tests.

Tests

Unit tests were deemed appropriate. No CI tests were added.

Review

Outstanding Questions

• 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