Support for `kong.log.set_serialize_value`

[renato-zannon]

Currently the plugin does not expose the kong.log.set_serialize_value function of the PDK, even though (as far as I can tell) it is the only way for a plugin to enrich the logs produced by file-log.

The go-pdk used to expose the function, but it was removed during the migration to protobuf.

[gszr]

Hey @lgtti, thank you for opening this issue! At the time the Go PDK was introduced, the kong.log.set_serialize_value PDK module did not exist; we recognize this as a desirable improvement, and welcome PRs - feel free to ask questions if you'd like to tackle it! Thanks once again - and happy Konging 🦍!