Placeholders in configuration

[jampukka]

Allow placeholders to be used in configuration. The main goal was to allow users to configure database connection settings as environment variables as requested in #50.

HakunapiConfigParser will now try to replace ${<placeholder>} snippets in property values by checking if an

1. environment variable or a
2. system property with that placeholder name exists.

If the placeholder value can't be resolved it's left as is. This is done because some next step might expect the value remain as a placeholder, for example the server url config added in #63.

Additionally, allow postgis database configurations to be inlined in hakunapi main configuration file, e.g.:

db=my_db
db.my_db.dataSourceClassName=org.postgresql.ds.PGSimpleDataSource
db.my_db.dataSource.user=address_reader
db.my_db.dataSource.password=test
db.my_db.dataSource.databaseName=address_fin
db.my_db.dataSource.portNumber=${DB_PORT}
db.my_db.dataSource.serverName=localhost

collections.addresses.db=my_db

Everything under db.<database_identifier>.* is passed on to HikariCP as-is (as in with the db.<database_identifier>. prefix removed). The end result is identical to a separate <database_identifier>.properties file.

[jampukka]

After a night's sleep two issues remain:

1. separate <my_db>.properties config files don't get treated the same as the main configuration file (this should be an easy fix)
2. if I understand az passwordless example correctly azure-identity-extensions might expect the ${AZ_ENV_VARIABLE} placeholders as placeholders (e.g. user=${AZ_POSTGRESQL_AD_NON_ADMIN_USERNAME} not user=my_datatabase_user). However, I don't see why it wouldn't work if hakunapi "resolved" the values before handing them to azure-identity-extensions

[nls-jajuko]

Approved. Inlined db properties is a good addition.

[jampukka]

1. separate <my_db>.properties config files don't get treated the same as the main configuration file (this should be an easy fix)

Fixed in 1acefa4