Nothing in core, but there are products that have good online karma (at least I haven't read horror stories, the lack of which is always good news). See https://en.wikipedia.org/wiki/Multi-master_replication#Postg... for some examples.

If your data has an obvious shard key, like customer ID, then I suggest taking a look at Citus. Not exactly multi-master, but a good sharded solution for PG. There is a managed version of Citus on Azure too.