feat: implement plugin config variable substitution in migrations

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