feat(structure-comparison): integrate structural comparison into the service layer

[PeachThinking]

What type of PR is this?

type-feature
module-structure-comaprison

What this PR does / why we need it:

1. structure comparison access to flowable layer: implement DBStructureComparisonFlowableTask to create a structure comparison task.

2. FlowInstanceController#getDownloadUrl adapted for downloading sql script generated by structure-comparison task.

3. structure comparison storage layer implementation.

4. add DefaultDBStructureComparatorTest for ob oracle mode.

5. Fix several ob oracle generate update table ddl bug:

• fix generate create partition ddl for HASH and KEY paritition type which do not need DBTablePartitionEditor#appendDefinitions, correct ddl is like:

PARTITION BY { HASH(expr) | KEY([column_name_list]) } 
    PARTITIONS partition_count;

• Filter out primary key index which will be compared as a constraint in ob oracle mode when generate create table ddl. see DBTableStructureComparator#copySourceTable
• Filter out not null check constraints which will be compared in column comparison in oracle mode. see DBTableStructureComparator#filterNotNullCheckConstraint
• Fix Map<String, List<DBTableConstraint>> listTableConstraints(String schemaName) only get one constraint in ob oracle mode .

6. Optimize ob mysql and ob oracle's method of querying all table objects under a schema. Different optimization logic is placed in the db-browser layer: DBSchemaAccessor#getTables.

• add a new method to optimize ob mysql list table indexes to avoid repeated query table ddl in OBMySQLSchemaAccessor as a public method.

Map<String, List<DBTableIndex>> listTableIndexes(String schemaName, Map<String, String> tableName2Ddl);

• add add a new method to optimize ob oracle get table ddl to avoid repeated query index ddl in OracleSchemaAccessor as a public method:

String getTableDDL(String schemaName, String tableName, List<DBTableColumn> tableColumns,
            List<DBTableIndex> tableIndexes);

Which issue(s) this PR fixes:

Fixes #816

Special notes for your reviewer:

Additional documentation e.g., usage docs, etc.:

[yhilmare]

pr naming suggestion: integrate structural comparison into the service layer

[yhilmare]

lgtm

[PeachThinking]

fix by #1473