babelfish-for-postgresql / babelfish_extensions / 12350160541

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Fix the nvarchar-varbinary casting (#3072)

PROBLEM: while casting nvarchar to varbinary we were considering the UTF8 encoding as input encoding in Babelfish
where as in TSQL we use UTF16 encoding fir nvarchar irrespective of input encoding.

RCA: we were considering varchar and nvarchar as same, whereas we should use input encoding for varchar and UTF16
encoding for nvarchar.

FIX: So we need to identify that if the input is nvarchar then we will do the UTF16 encoding.

For a casting like nvarchar->varbinary->nvarchar, now since for the casting we are encoding the input string into UTF16
encoding via function nvarcharvarbinary, so while converting varbinary-> nvarchar we will use the function
varbinarynvarchar where we will convert UTF16 encoding to UTF8 with null padding.

So we created a function nvarcharvarbinary and varbinarynvarchar to handle nvarchar<-> varbinary to and fro casting.
And for this casting we have specifically applied a condition that we will not convert the datatype to basetype before choosing the casting function

Task: BABEL-4891
Signed-off-by: Pranav Jain <pranavjc@amazon.com>

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