Feat/1679 make dlt init use core sources

[sh-rp]

Description

Updates the dlt init command to use the core source if present. It uses all the same mechanisms as before, so secrets extractions and destination replacements still work.

What this PR does so far:

• [ ] User core sources if present (filesystem for now)
• [ ] Move generic pipeline to core
• [ ] Allows for omitting core sources
• [ ] Updates wording in a hopefully useful way
• [ ] Add a note about setting up the custom destination in the correct way
  • [ ] Update the command that lists the available sources to show the core sources also

Follow up ToDos:

• [ ] Update generic pipeline template and add test for it (or figure out what templates there should be)
• [ ] Allow modifiers for sources, e.g. dlt init filesystem:az duckdb will create credentials for azure

[rudolfix]

@sh-rp let's move forward! I'm particularly interested how you'll deal with more complex templates. currently our demo pipelines are runnable code but in the core they do not need to be... so we do not need to use AST to rewrite them.