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

首先跟踪必需的环境字段 source-kind 是如何配置并添加到发出的事件中的。比较现有的环境和配置文件处理方式，然后在保留事件发出的同时，将该流程替换为支持 meta；当可以通过受支持的配置路径提供元数据，并且不再需要 source-kind 时，即表示完成。

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.