[cosn] Log Hadoop config entries at debug level instead of warn

[thswlsqls]

Purpose

fix #8553

• COSNFileIO.configure() logged each fs.cosn.* entry with LOG.warn; change it to LOG.debug.
• Restores parity: the five sibling filesystem impls (oss/obs/s3/azure/jindo) all log this identical "Adding config entry" statement at LOG.debug; COSN was the outlier.
• Side benefit: the fs.cosn.userinfo.secretKey value is no longer emitted at the default WARN level (siblings also log the value, but only at debug — this change aligns the level only, it does not mask the value).

Tests

• No unit test added: log-level assertions are not idiomatic in Paimon (no sibling filesystem impl tests configure() log levels, and paimon-cosn-impl has no test source dir).
• Verified by mvn -pl paimon-filesystems/paimon-cosn-impl -am -DfailIfNoTests=false clean install (JDK 11) — BUILD SUCCESS, including spotless-check, checkstyle, and rat.