Where to save data used by extension ?

Hi pgsql-hackers,

I have an extension for postgresql. The extension works across some
databases, and needs to save some data. The data might be modified
dynamically by the extension, and all the changed result must be saved. I
have considered some methods.
1. Use GUC
I find no easy way to write the GUC values back to postgres.conf. And the
data might be a bit complex
2. Create a table under a special database, like postgres
The weak is the strong assumption that the database postgres does exist and
will not be dropped.
3. Create a table under a database created by the extension
A little heavy, but without dependencies.
4. Write to a native file.
The file can not sync to a standby

Currently, I use #2. Is there any better way to do this?

Thank you very much
Best Regards