Convert events metadata type to google.protobuf.Struct

[tomhjp]

The bytes type gets seralised as base64 by protojson when marshalled into JSON, whereas google.protobuf.Struct is the equivalent of a JSON object, so gives us a bit of structure, but still plenty of flexibility that fits in with common models of metadata, as well as nice human-readable serialisation over JSON.

As JSON before:

{"event":{"id":"323c94b3-2638-1e2b-cfd8-2b135b369400","metadata":"eyJjdXJyZW50X3ZlcnNpb24iOiIyIiwib2xkZXN0X3ZlcnNpb24iOiIwIiwicGF0aCI6ImRhdGEvZm9vIn0="},"eventType":"kv-v2/data-write","pluginInfo":{"mountClass":"secret","mountAccessor":"kv_18df2fbe","mountPath":"secret/","plugin":"kv"},"timestamp":"2023-02-10T11:14:24.899815Z"}

As JSON after:

{"event":{"id":"cb003f60-17b8-b299-d71b-082adda7fdd3", "metadata":{"current_version":"1", "oldest_version":"0", "path":"data/foo"}}, "eventType":"kv-v2/data-write", "pluginInfo":{"mountClass":"secret", "mountAccessor":"kv_e26c7797", "mountPath":"secret/", "plugin":"kv"}, "timestamp":"2023-02-10T12:26:58.558572Z"}

[swenson]

LGTM