Native functions w/ diff DBs for CAP Java

[rjayasinghe]

In order to use database dependent CDS models with CAP Java applications you need to consider some additional steps. Those are added to the database guide in this commit.

[renejeglinsky]

If I understand it correctly, this is about using two different DBs locally. If it's only one DB locally and one DB (HANA, Postgres) for production, then you can stay with the defaults, right?

Suggested change

3. For CAP Java setups you might need to reflect the different profiles in your CDS Maven plugin configuration. This might not be needed for all setups. For mixed local DB (sqlite, H2, or PostgreSQL) and hybrid HANA setup nothing special needs to be done since the local build defaults to the `development` profile. But for other setups like e.g. local PostgreSQL and SQLite you'll need two (profiled) `cds deploy` commands:

3. For CAP Java setups you might need to reflect the different profiles in your CDS Maven plugin configuration. This might not be needed for all setups, like using a standard local database (sqlite, H2, or PostgreSQL) and a production SAP HANA setup. In that case the local build defaults to the `development` profile. But for other setups, like using a local PostgreSQL and a local SQLite you'll need two (profiled) `cds deploy` commands:

[renejeglinsky]

If my assumption (two local DBs) is correct, why is the production profile needed here?

[renejeglinsky]

I guess we should not promote the default-env.json case anymore when we have the more secure cds bind. But it's not started using cds bind, right? We need to rephrase that.
I removed some parts as I thought that is too detailed and probably not needed. But of course you can change my mind ;)

Suggested change

4. For the Spring Boot side it's similar. If you have a local development DB and a hybrid profile with a remote HANA DB you only need to run in default (or any other) profile and you get the local DB and it's SQL schema that was written to your `src/main/resources`. For the HANA part the build and deploy part is done seperately and the application just needs to be started with a local `default-env.json` file and `cloud` profile or via `cds bind`.

4. For the Spring Boot side it's similar. If you have a local development database and a hybrid profile with a remote SAP HANA database, you only need to run in default (or any other) profile. For the SAP HANA part, the build and deploy part is done separately and the application just needs to be started using `cds bind`.