Implicit data_stream.dataset variable in input packages

It should be documented what implicit variables are available to handlebar templates for input packages. Input packages appear to be able to reference an `{{data_stream.dataset}}` despite it not being declared within any manifest. We should document this so that developers can discover its existence through the package-spec. For example:

https://github.com/elastic/package-spec/blob/48461578910c405fcc6a92b97a1e558aeb8fb860/test/packages/custom_logs/agent/input/input.yml.hbs#L18

https://github.com/elastic/integrations/blob/80579aa022841f78d5020a274ef103249f6af810/packages/sql_input/agent/input/input.yml.hbs#L14-L15

It should be noted that some input packages do explicitly declare the variable (maybe they shouldn't be doing this?).