Delay JSON serialization until delivery when using the thread queue delivery method

[imjoehaines]

Goal

This PR adds a new method that delivery methods can implement in order to delay serialization of the Bugnsag::Report object. This is used by the thread queue delivery method to shift some work off of the main thread

This works by passing a proc that will serialize the Report rather than the serialized Report itself, this way the delivery method doesn't have to know how things are serialized only how to get the serialized value. This is a bit less flexible than passing the Report directly, but I don't think it makes sense for delivery methods to use different serialization

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• 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

For the pull request reviewer(s), this changeset has been reviewed for:

• 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