Make connector task partition readability for logs

Add toString method for connector task partitions.
log info:
Found previous partition offset MySqlPartition [sourcePartition=\{server=mysql_local}]: MySqlOffsetContext [sourceInfoSchema=Schema{io.debezium.connector.mysql.Source:STRUCT}, sourceInfo=SourceInfo [currentGtid=null, currentBinlogFilename=binlog.000010, currentBinlogPosition=4178, currentRowNumber=0, serverId=0, sourceTime=null, threadId=-1, currentQuery=null, tableIds=[], databaseName=null], snapshotCompleted=false, transactionContext=TransactionContext [currentTransactionId=null, perTableEventCount={}, totalEventCount=0], restartGtidSet=bc7a69e6-4216-11ec-83d6-16bf7d36954a:1-55, currentGtidSet=bc7a69e6-4216-11ec-83d6-16bf7d36954a:1-55, restartBinlogFilename=binlog.000010, restartBinlogPosition=4178, restartRowsToSkip=1, restartEventsToSkip=2, currentEventLengthInBytes=0, inTransaction=false, transactionId=null, incrementalSnapshotContext =IncrementalSnapshotContext [windowOpened=false, chunkEndPosition=null, dataCollectionsToSnapshot=[], lastEventKeySent=null, maximumKey=null]] (io.debezium.connector.common.BaseSourceTask:313)