Add PostgreSQL module

[pamelafox]

This PR adds a module to /core for PostGreSQL flexible servers. I used sqlserver.bicep as inspiration, and ported over a simple Flask+PG app to use the module. You can see my usage here:
https://github.com/pamelafox/flask-db-quiz-example/blob/765f1a0abf4da055ac910a520ed8f26d0a076cd4/infra/main.bicep#L31

Design decisions made:

• What to call the parameters: I typically called them the same thing they're called in the actual Bicep, as that seems the most obvious choice.
• What to turn into parameters: I made SKU, storage, database name, login details, and PG version into params. That's inspired by what you customize in the portal and by what sqlserver.bicep parameterizes. It might also make sense to make highAvailability mode a parameter, as that's something that the Portal lets you specify, but none of my samples need it.
• What properties to specify/leave out: I created a new PG server in the Portal, looked at its properties to figure out what the defaults are, and removed properties that were already the default values. I wasn't sure if High Availability is default disabled or not, since that's part of the PG server creation process, so I left it in as an explicit property.

TODO:

• Make sure it's compatible with https://github.com/Azure-Samples/bindings-dapr-nodejs-cron-postgres/blob/main/infra/core/postgres.bicep
• Add support for VNet connection (In follow-up PR)

[pamelafox]

I ported another template today and had to make some changes to this to allow for Virtual Network support. That's going to be hairier so I don't have it ready yet. FYI in case you want to wait on review, or want that to be a follow-up PR. (It'll be an addition more than a change).

[jongio]

@weikanglim - Please merge when ready (we'll need to merge for folks that aren't in azdcore)