docs: add decision record about using PostgreSQL API (#26)

Author: paullatzelsperger

docs/developer/decision-records/2023-07-12_use_cosmos_postgres_api/README.md

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+# Use the PostgreSQL API for CosmosDB
+
+## Decision
+
+EDC will use the PostgreSQL API for CosmosDB instead of Cosmos' own client SDK ("SQL API").
+
+## Rationale
+
+Now that Cosmos fully supports PostgreSQL (https://learn.microsoft.com/en-us/azure/cosmos-db/postgresql/introduction), there is no need to keep the CosmosDB client around. CosmosDB can now be targeted using the JDBC PostgreSQL driver. 
+This will greatly reduce the maintenance surface, and it may even reduce test runtime.
+
+These store implementations do not yet have a PostgreSQL variant:
+- `FederatedCacheNodeDirectory`
+
+
+## Approach
+
+- delete all specific CosmosDB implementations for persistence, such as `AssetIndex`, etc.
+- delete the `CosmosFederatedCacheNodeDirectory`, as it is not used currently
+- for our integration tests, we'll need to provision a "Cosmos Postgres cluster" and retire our existing CosmosDB
+- for each store implementation, add tests in the `Technology-Azure` repo targeting a CosmosDB PostgreSQL cluster specifically, for example:
+  ```java
+  @ComponentTest
+  @ExtendWith(EdcExtension.class)
+  public class CosmosPostgresTest extends AssetIndexTestBase {
+      private final BaseSqlDialectStatements sqlStatements = new PostgresDialectStatements();
+      private final QueryExecutor queryExecutor = new SqlQueryExecutor();
+      private SqlAssetIndex sqlAssetIndex;
+      private NoopTransactionContext transactionContext;
+      private DataSource dataSource;
+
+      @BeforeEach
+      void setUp()  {
+          var typeManager = new TypeManager();
+          typeManager.registerTypes(PolicyRegistrationTypes.TYPES.toArray(Class<?>[]::new));
+
+          var dsName = "test-ds";
+          var reg = new DefaultDataSourceRegistry();
+          dataSource = createDataSource();
+          reg.register(dsName, dataSource);
+
+          System.setProperty("edc.datasource.asset.name", dsName);
+
+          transactionContext = new NoopTransactionContext();
+          sqlAssetIndex = new SqlAssetIndex(reg, dsName, transactionContext, new ObjectMapper(), sqlStatements, queryExecutor);
+
+          var schema = Files.readString(Paths.get("docs/schema.sql"));
+          runQuery(schema);
+      }
+
+      @AfterEach
+      void tearDown() {
+          runQuery("DROP TABLE " + sqlStatements.getAssetTable() + " CASCADE");
+          runQuery("DROP TABLE " + sqlStatements.getDataAddressTable() + " CASCADE");
+          runQuery("DROP TABLE " + sqlStatements.getAssetPropertyTable() + " CASCADE");
+      }
+
+      @Override
+      protected SqlAssetIndex getAssetIndex() {
+          return sqlAssetIndex;
+      }
+
+      private DataSource createDataSource() {
+          var ds = new PGSimpleDataSource();
+          // this can be obtained after setting up the Cosmos-PG cluster
+          // should be injected through environment variables
+          ds.setServerNames(new String[]{ "c-edc-pg-test-cluster.pnrboctaun4gkt.postgres.cosmos.azure.com" });
+          ds.setPortNumbers(new int[]{ 5432 });
+          ds.setUser("<USER>");
+          ds.setPassword("<PASSWORD>");
+          ds.setDatabaseName("<DBNAME>");
+          ds.setSslmode("require");
+          return ds;
+      }
+
+      private void runQuery(String schema) {
+          try (var connection = dataSource.getConnection()) {
+              transactionContext.execute(() -> queryExecutor.execute(connection, schema));
+          } catch (SQLException e) {
+              throw new RuntimeException(e);
+          }
+      }
+  }
+  ```