event-plugin: Replace source enum with metadata field in events plugin

# Problem
Right now we have a required env var field `source-kind` that is related to the LSP architecture. This field is added to an every event emitted and helps to distinguish between different node types within LSP architecture.

# Solution
Since we want the plugin to be more generic, we need to get rid from `source-kind`, however still have an ability to add additional information to an event. Therefore, we'll benefit from a new field called 'meta':
- `meta` should be set via env variables. We can go in both ways of providing meta data:
- env variables. We can define an environment variable which would receive metadata as a key-value string. e.g. this could be in JSON format, something like `EVENT_PLUGIN_META='{"source_kind": "gateway"}'`;
- config file. We can have a config file (e.g. `.toml`) which would contain an information. That's might suit us well since we already have a config file;
- apply both variants;
- the flow should be the same as it was introduced for `source-kind`.