feat: add method to add metadata section values as a dictionary (restarted)

[robinmacharg]

[Note: this PR replaces #456 (Branch here) which was proving difficult to merge. The PR text is largely unchanged.]

Goal

Adds a BugsnagMetadata.addMetadataToSection:values: method to allow merging arbitrary dictionaries with preexisting Metadata.

Design

Simple fast enumeration loop over provided values with appropriate checks for:

• Validity of Key (String subclass)
• Validity of Value (can be sanitised: a recursive check that all (nested) values are of type Number, String, Array, Dictionary or Null

Conditional addition/deletion depending on whether value is null or not. There is a check to ensure that empty sections are not created for (failing) invalid values. Invalid values are logged and ignored. Delegate callback is called only where there is a material - CRUD - change to the metadata.

Changeset

• Method added to BugsnagMetadata.
• Unit Tests added.

Tests

• Added Unit tests only. This change affects the framework API and doesn't change any external aspect.
• CI passes.

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