Adding re-usable data in functions.php or plugin

I am creating a network of web sites very soon and each one of these web sites will have data specific to it, but each web site will need the same.

Items include:

Postal address
Phone number
Email
Google maps API key
Analytics code
and more..

Is it possible to add this in the form of a plugin, and thus add this data when logged in, so that I can code my template to present this data with php?

Or would adding it to my functions.php file be better?

The whole aim of this is to edit one file, or add this data in one location, so I can use it in multiple instances across a web site, rather than typing it out again and again.

I would love to have a plugin called “common features” or something, that has labels and input fields where I type this data in once, and then have my template call this data whenever it needs to, either in shortcodes or php.

How would I do this?

Thanks in advance!