Add DECLARE STATEMENT support to ECPG.

Add DECLARE STATEMENT support to ECPG.

DECLARE STATEMENT is a statement that lets users declare an identifier
pointing at a connection.  This identifier will be used in other embedded
dynamic SQL statement such as PREPARE, EXECUTE, DECLARE CURSOR and so on.
When connecting to a non-default connection, the AT clause can be used in
a DECLARE STATEMENT once and is no longer needed in every dynamic SQL
statement.  This makes ECPG applications easier and more efficient.  Moreover,
writing code without designating connection explicitly improves portability.

Authors: Ideriha-san ("Ideriha, Takeshi" <ideriha.takeshi@jp.fujitsu.com>)
         Kuroda-san ("Kuroda, Hayato" <kuroda.hayato@jp.fujitsu.com>)

Discussion: https://postgr.es/m4E72940DA2BF16479384A86D54D0988A565669DF@G01JPEXMBKW04