Add --datasource.* CLI flags to replace DATA_SOURCE_* env vars

Part of #1371. **Summary** `GetDataSources()` in `exporter/datasource.go` currently reads the Postgres connection config only from env vars, with no CLI flag path. Add flags that provide the same configuration options, so the env vars have something to deprecate in favor of. **New flags** - `--datasource.dsn` (replaces `DATA_SOURCE_NAME`) - `--datasource.uri` (replaces `DATA_SOURCE_URI`) - `--datasource.uri-file` (replaces `DATA_SOURCE_URI_FILE`) - `--datasource.user` (replaces `DATA_SOURCE_USER`) - `--datasource.user-file` (replaces `DATA_SOURCE_USER_FILE`) - `--datasource.pass` (replaces `DATA_SOURCE_PASS`) - `--datasource.pass-file` (replaces `DATA_SOURCE_PASS_FILE`)