Make metadata interface consistent across config client and event

[robinmacharg]

Goal

Implement a consistent metadata interface across Config, Client and Event classes.

Design

A BugsnagMetadataStore instance-level protocol is defined, as well as a class-level version of the same, containing the following methods:

• addMetadata:toSection:
• addMetadata:withKey:toSection:
• getMetadataFromSection:
• getMetadataFromSection:withKey:
• clearMetadataFromSection:
• clearMetadataFromSection:withKey:

Bugsnag, BugsnagClient, BugsnagEvent, BugsnagConfig and BugsnagMetadata classes are all changed to conform to this.

Internal dictionary metadata implementations are replaced with BugsnagMetadata objects.

Changeset

A wide range of files, not limited to those mentioned above, since many references to object metadata also needed updated. Test apps have also been updated.

Tests

No significant new tests added, but many changes to existing ones to adopt the new non-dictionary access syntax.

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

[fractalwrench]

Happy with the overall plan, and the public signature changes made in this changeset.

Making BugsnagMetadata conform to the protocol would be good, as the implementation for adding/getting/clearing metadata could be contained within there. BugsnagClient and BugsnagEvent could then hold a property of type BugsnagMetadata and forward on any method calls to it, rather than implementing each call in multiple classes. We've done something similar on Android here.

[fractalwrench]

This looks good - and is a hard thing to change. I've left a few inline comments but think this is basically there and will be ready for merging once they've been resolved.