Provide better support for imports at the plugin level

[smaye81]

Currently, the Buf CLI provides options for include-imports and include-wkt, which handles the generation of imports and WKT.

      --include-imports        Also generate all imports except for Well-Known Types.
      --include-wkt            Also generate Well-Known Types. Cannot be set without --include-imports.

However, because this is at the command level, this will be applied to every plugin in buf.gen.yaml, which works fine for some languages (such as JS) but can be problematic for languages such as Go because of how go_package works.

A better approach might be to move these commands inside buf.gen.yaml and scoped to individual plugins. That way, each plugin can be told what files to generate code for independently.

For further context and a real-world example of the difficulties this causes, see connectrpc/connect-es#262.

[bufdev]

We might want to re-think buf.gen.yaml in a v2 of the config definition. Originally, the motivation was to have buf.gen.yaml be input-independent - all input-related functionality was in flags, all input-independent plugin config was in buf.gen.yaml. This would let you reuse buf.gen.yamls across different inputs.

Real-world usage has not played out this way - most people check a buf.gen.yaml into the root of their repository. Managed mode also blurred the lines. We may want to have an updated buf.gen.yaml that allows inputs to be specified.

[bufdev]

Merging this into #1424