feat: add nullable civil types for date/time parameters

[dlevy-msft-sql]

Adds NullDate, NullDateTime, and NullTime types that wrap civil.Date, civil.DateTime, and civil.Time with nullable semantics.

Changes

• civil_null.go: New NullDate, NullDateTime, NullTime types with Scanner and Valuer interfaces
• mssql_go19.go: Handle nullable civil types in convertInputParameter
• mssql.go: Handle null encoding in makeParam
• tvp_go19.go: TVP support for nullable civil types

Features

• Implements Scanner and Valuer interfaces for database operations
• Supports TVP (Table Value Parameters) with null handling
• Supports OUT parameters

Closes #289

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.34615% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.36%. Comparing base (95e5e8a) to head (23d7c2c).

Files with missing lines	Patch %	Lines
tvp_go19.go	0.00%	21 Missing ⚠️
mssql.go	0.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #325      +/-   ##
==========================================
- Coverage   78.59%   78.36%   -0.24%     
==========================================
  Files          34       35       +1     
  Lines        6598     6702     +104     
==========================================
+ Hits         5186     5252      +66     
- Misses       1134     1171      +37     
- Partials      278      279       +1     

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