CDAP-15551 DB2 db plugin enhancements: all data types support and proper test coverage

[vladhlinskiy]

JIRA: https://issues.cask.co/browse/CDAP-15551
WIKI: https://wiki.cask.co/display/CE/DB2+database+plugin

In scope of this PR:

• Added tests for Source plugin for next types:

VARGRAPHIC, DBCLOB, BINARY, VARBINARY

• Added tests for Sink plugin for next types:

SMALLINT, INTEGER, BIGINT, DECIMAL(p,s) and NUMERIC(p,s), DECFLOAT, REAL, DOUBLE, CHAR, CHAR FOR BIT DATA, VARCHAR FOR BIT DATA, BINARY, VARBINARY, GRAPHIC, VARGRAPHIC, DBCLOB

• DECFLOAT is mapped to the Type.STRING

• Mappings added to the reference documentation

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[vladhlinskiy]

@albertshau could you, please, take a look?

[albertshau]

lgtm