FLIP-84: Improve & Refactor API of TableEnvironment & Table

TableEnvironment.sqlUpdate(String)
TableEnvironment.insertInto(String, Table)
TableEnvironment.execute(String)
TableEnvironment.explain(boolean)
TableEnvironment.fromTableSource(TableSource<?>)
Table.insertInto(String)

interface TableEnvironment {
    // execute the given single statement, and return the execution result.
    TableResult executeSql(String statement);
     
    // get the AST and the execution plan for the given single statement (DQL, DML)
    String explainSql(String statement, ExplainDetail... extraDetails);
 
    // create a StatementSet instance which can add DML statements or Tables
    // to the set and explain or execute them as a batch.
    StatementSet createStatementSet();
}

interface Table {
    // write the Table to a TableSink that was registered
    // under the specified path.
    TableResult executeInsert(String tablePath);
     
    // write the Table to a TableSink that was registered
    // under the specified path.
    TableResult executeInsert(String tablePath, boolean overwrite);
 
    // create a StatementSet instance which can add DML statements or Tables
    // to the set and explain or execute them as a batch.
    String explain(ExplainDetail... extraDetails);
 
    // get the contents of the current table.
    TableResult execute();
}

interface TableResult {
     // return JobClient if a Flink job is submitted
     // (for DML/DQL statement), else return empty (e.g. for DDL).
    Optional<JobClient> getJobClient();
 
    // return the schema of the result
    TableSchema getTableSchema();
     
    // return the ResultKind which can avoid custom parsing of
    // an "OK" row in programming
    ResultKind getResultKind();
 
    // get the row contents as an iterable rows
    Iterator<Row> collect();
 
    // print the result contents
    void print();
}

public enum ResultKind {
    // for DDL, DCL and statements with a simple "OK"
    SUCCESS,
 
    // rows with important content are available (DML, DQL)
    SUCCESS_WITH_CONTENT
}

interface StatementSet  {
    // add single INSERT statement into the set
    StatementSet addInsertSql(String statement);
 
    // add Table with the given sink table name to the set
    StatementSet addInsert(String targetPath, Table table);
 
    // add Table with the given sink table name to the set
    StatementSet addInsert(String targetPath, Table table, boolean overwrite);
 
    // returns the AST and the execution plan to compute
    // the result of all statements and Tables
    String explain(ExplainDetail... extraDetails);
 
    // execute all statements and Tables as a batch
    TableResult execute();
}

public enum ExplainDetail {
   STATE_SIZE_ESTIMATE,
   UID,
   HINTS,
   ...
}