It is now possible to write in migration `@_plugin_config.foo` where foo is
variable from plugin configuration. Note that configuration typically
belongs to individual services, in catalog this field is called entity. For
migration special values there is special entity called `migration_context`.
So corresponding alter looks this way:

ALTER PLUGIN p 0.1.0 SET migration_context.foo = '"nondefault"'

(extra quotes are temporary, and should be dealt with separately)

Closes https://git.picodata.io/picodata/picodata/picodata/-/issues/659

Previouslly we've blocked on scope termination before reading from
channel, so blocking still occured.

With this patch there is new `blocking` wrapper function that executes
provided closure on separate thread and yields until result is ready

picoplugin -> picodata-plugin
picoplugin_gen -> picodata-plugin-proc-macro