add --with-prelude option to sea-orm-cli

[hahouari]

I opened a discussion on this feature a month ago (link below), I suppose it didn't take notice, so I picked it up and implemented the feature myself on my free time, and I'm happy to contribute it back :).

PR Info

• Closes generate entities without prelude [sea-orm-cli] #2283

New Features

Added --with-prelude option to sea-orm-cli generate entity command, it accepts 3 values:

• [1] all: the default value, cli tool behaves as it does right now (so no breaking changes), it will generates the prelude.rs file and add it to mod.rs (or lib.rs).
• [2] none: it WILL NOT generates the prelude.rs file and it WILL NOT add it to mod.rs.
• [3] all-allow-unused-imports: it generates the prelude.rs file and add it to mod.rs, BUT the following inner attribute #![allow(unused_imports)] is appended to prelude.rs heading, see below:

This is how it looks like with --with-prelude=all:

This is how it looks like with --with-prelude=all-allow-unused-imports: