[ADQL,TAP] Add UDF support in the TAP configuration file.

src/adql/db/DBChecker.java

src/adql/db/FunctionDef.java

src/adql/query/operand/function/DefaultUDF.java

src/adql/query/operand/function/UserDefinedFunction.java

src/adql/translator/JDBCTranslator.java

src/tap/config/DefaultServiceConnection.java

src/tap/config/TAPConfiguration.java

src/tap/config/tap_configuration_file.html

test/adql/db/TestDBChecker.java

test/tap/config/TestDefaultServiceConnection.java