Implicit data_stream.dataset variable in input packages

[andrewkroh]

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:

package-spec/test/packages/custom_logs/agent/input/input.yml.hbs

Line 18 in 4846157

dataset: {{data_stream.dataset}}

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?).