feat(pgwire): support 1-dimension array in extended mode

[fuyufjh]

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What's changed and what's your intention?

Related #7949

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[kwannoel]

Core functionality LGTM.

Adding some notes for other reviewers who are not as familiar with PG coding conventions (like myself), so it's easier for them to review it.

The core functionality is specifically in these lines:
https://github.com/postgres/postgres/blob/c1c09007e219ae68d1f8428a54baf68ccc1f8683/src/backend/utils/adt/arrayfuncs.c#L1606-L1644.

Some things to take note of:

			pq_sendint32(&buf, VARSIZE(outputbytes) - VARHDRSZ);

Here this just gets the length of outputbytes. VARSIZE gets the size of varlena. VARHDRSZ is the header len of varlena. Subtracting them gives us the data len.

AARR_DIMS gives us the dimension of each array (i.e. the len).
AARR_LBOUND gives us the starting offset for each array.

[xiangjinwu]

SQL array starts at 1 rather than 0.

test=# select array_send(array[2, 3, null]);
                                     array_send                                     
------------------------------------------------------------------------------------
 \x000000010000000100000017000000030000000100000004000000020000000400000003ffffffff
(1 row)