Implement a new OPAQUE metadata type

[lemnik]

Goal

Introduce a new OPAQUE metadata value type to allow complex metadata values to be stored by the NDK layer for later delivery. This PR introduces the type enum, fields to store the values within, and migrations for existing event files. It does not include any of the encoding or decoding logic.

Changeset

• Add BSG_METADATA_OPAQUE_VALUE as a possible metadata value type
• Add opaque_value and opaque_value_size to the bsg_metadata_value struct for storage
• Bump the size limit of metadata value section and name to 64 chars
• Migrate all existing report formats to a new version (9) with the new structure

Testing

New test for the latest binary event format, and updated all existing migration tests to cover the change to metadata and breadcrumbs (which also contain metadata).

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1862.77	1610.5
arm64_v8a	655.76	405.9
armeabi_v7a	590.23	340.37
x86	729.47	479.61
x86_64	696.71	446.85

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