fix(pg-cdc): handle citext data type and other string compatible data types

[StrikeW]

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What's changed and what's your intention?

Support mapping citext data type and other string compatible data types

ref: link

close: #18844

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[xiangjinwu]

We shall warn users, at least in the doc, that these data types would lose their original semantic after being ingested as varchar. For example,

'foo'::char(5) = 'foo '::char(5)
'foo'::varchar <> 'foo '::varchar