feat: Java API to configure named connections

[hessjcg]

In real usage, an application would first call Connector.register() to register the configuration for a named connection.

ConnectorConfig config =  new ConnectorConfig.Builder()
            .withTargetPrincipal("example@project.iam.googleapis.com")
            .withDelegates(Arrays.asList("delegate@project.iam.googleapis.com"))
            .build();
ConnectorRegistry.register("my-connector", config);

Then the application would use the connector by adding the cloudSqlNamedConnector parameter to the JDBC URL:

String jdbcUrl = "jdbc:mysql:///<DATABASE_NAME>?"+
  + "cloudSqlInstance=project:region:instance"
  + "&cloudSqlNamedConnector=my-connector"
  + "&socketFactory=com.google.cloud.sql.mysql.SocketFactory"
  + "&user=<DB_USER>&password=<PASSWORD>";

Or using JDBC connection properties, by adding the cloudSqlNamedConnector property:

// Set up URL parameters
Properties connProps=new Properties();
connProps.setProperty("user","<DB_USER>");
connProps.setProperty("password","<PASSWORD>");
connProps.setProperty("sslmode","disable");
connProps.setProperty("socketFactory","<DRIVER_CLASS>");
connProps.setProperty("cloudSqlInstance","my-connection");
connProps.setProperty("cloudSqlNamedConnector","[my-connection](project:region:instance)");

// Initialize connection pool
HikariConfig config=new HikariConfig();
config.setJdbcUrl("jdbc:mysql:///<DB_NAME>");
config.setDataSourceProperties(connProps);
config.setConnectionTimeout(10000); // 10s

HikariDataSource connectionPool=new HikariDataSource(config);

The application can stop individual named connectors, or all connectors:

ConnectorRegistry.close("my-connector");

The application can shutdown the connector, preventing any new connections
and stoping all connector threads.

ConnectorRegistry.shutdown();

Fixes #1226

[hessjcg]

Converting back to draft. This got too big so I split it into several PRs:

• chore: Better name for the lock field in Refresher #1671
• feat: make ConnectorConfig part of the public api, move ConnectionConfig to internal #1672
• test: Refactor InternalConnectorRegistryTest to remove redundant test code #1673

[enocom]

Two things and then we can merge this:

• Let's make sure to close all unnamed connectors
• The tests of named connectors aren't clear in how they're confirming we're using a named connector.

[enocom]

nit else is unnecessary after a return

[hessjcg]

Fixed.

[enocom]

same nit here.

[hessjcg]

Fixed.

[enocom]

nit: could this be all in one line?

[hessjcg]

Fixed.

[enocom]

We need to close all the unnamed connectors in the connectors map.

[hessjcg]

Fixed.

[enocom]

Sorry I still don't understand -- how do we know the registered connector was used and not simply an unnamed connector?