Fix the nvarchar-varbinary casting

[pranavJ23]

Description

Cherry-Pick: #3072
PROBLEM: while casting nvarchar to varbinary we were considering the 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

Issues Resolved

[List any issues this PR will resolve]

Test Scenarios Covered

• Use case based -

• Boundary conditions -

• Arbitrary inputs -

• Negative test cases -

• Minor version upgrade tests -

• Major version upgrade tests -

• Performance tests -

• Tooling impact -

• Client tests -

Check List

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[coveralls]

Pull Request Test Coverage Report for Build 12387540169

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Details

• 146 of 181 (80.66%) changed or added relevant lines in 3 files are covered.
• 448 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 74.838%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
contrib/babelfishpg_common/src/varbinary.c	83	91	91.21%
contrib/babelfishpg_common/src/varchar.c	40	67	59.7%

Files with Coverage Reduction	New Missed Lines	%
contrib/babelfishpg_tsql/src/pl_handler.c	53	90.05%
contrib/babelfishpg_tsql/src/pltsql_coerce.c	112	82.47%
contrib/babelfishpg_tsql/src/hooks.c	283	83.91%

Totals
Change from base Build 12352168391:	0.02%
Covered Lines:	46618
Relevant Lines:	62292

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💛 - Coveralls