chg: dev: rename ns_pluginregister_ctx_t into ns_pluginctx_t

The type `ns_pluginregister_ctx_t` was initially added to pass plugin
contextual data when the plugin is registered, but this is also now
passed into `plugin_check`. Furthermore, those various data are not
specific to the registration in particular. Rename the type into
`ns_pluginctx_t` for clarity.

Merge branch 'colin/rename-pluginctx-rename' into 'main'

See merge request isc-projects/bind9!11035