Improve metadata API in Report/Event

[imjoehaines]

Goal

This PR adds two new methods to replace the existing add_tab/remove_tab — add_metadata and clear_metadata

The new methods support almost all the use-cases of the old methods, with some additional functionality that matches other notifiers (e.g. @bugsnag/js):

event.add_metadata(:abc, :xyz, 123) # => { abc: { xyz: 123 } }
event.add_metadata(:abc, { x: 1, y: 2, z: 3 }) # => { abc: { xyz: 123, x: 1, y: 2, z: 3 } }

# a value of 'nil' removes the key entirely
event.add_metadata(:abc, :xyz, nil) # => { abc: { x: 1, y: 2, z: 3 } }

# new values will overwrite old ones, 'nil' still removes the key entirely
event.add_metadata(:abc, { x: nil, z: 9 }) # => { abc: { y: 2, z: 9 } }

event.clear_metadata(:abc, :y) # => { abc: { z: 9 } }
event.clear_metadata(:abc) # => {}

The only use-case that add_tab supported that add_metadata doesn't is the automatic "custom" tab. This was created if add_tab was called with a non-Hash value, but is quite a surprising API (e.g. see #587):

event.add_tab(:abc, 123) # => { "custom" => { abc: 123 } }
# is equivalent to:
event.add_metadata("custom", :abc, 123) # => { "custom" => { abc: 123 } }

Design

The implementation uses a MetadataDelegate class internally. This is to allow the new metadata operations to be supported in multiple classes as Configuration will gain support for global metadata in the future