ref(service): Use JSON to encode metadata

[jan-auer]

Encode metadata using JSON instead of bincode. This fixes two issues:

1. Governance and evolution. We can now add and reorder metdata fields without
   risking to break past consumers.
2. Partial serialization: For cases where we need to partially serialize
   metadata, we can do so now. Bincode will fail when using skip on fields.

The implementation on LocalFs is not the most efficient one, since it allocates
a string for metdata. However, since our metadata tends to be fairly small and
we will place strict limits on the maximum number and size of custom fields,
this is OK.

This is a one-time breaking change.

Fixes FS-178

[linear]

FS-178 Check forward-compatibility of metadata serialization