Support option retention in @bufbuild/protoplugin

[timostamm]

Our plugin framework @bufbuild/protoplugin supports access to custom Protobuf options (see the documentation here). Currently, it does not support option retention yet (see the docs here, and the comments in plugin.proto).

We need to update @bufbuild/protoplugin to provide the descriptors that include RETENTION_SOURCE options so that users can access them when generating code. For users who do need the descriptors with stripped options (only RETENTION_RUNTIME), we have to provide alternate means.

[timostamm]

Also see #701.

[timostamm]

This was implemented in #827 for V2.

[timostamm]

This is implemented in version 2.

Generated schemas do not include options with retention = RETENTION_SOURCE.

Schema.files from @bufbuild/protoplugin always includes all options.