Add validation/warning for unsupported config file extensions

[dlevy-msft-sql]

Description:
Currently, when users specify a config file with an unsupported extension (e.g., --sqlconfig myfile.txt), they receive a confusing Viper error: "Unsupported Config Type 'txt'".

Proposed Enhancement:
Add validation in internal/config/viper.go to check the file extension before passing to Viper and provide a clearer error message such as:

Error: Configuration files must use YAML format with .yaml or .yml extension.
The file 'myfile.txt' has an unsupported extension '.txt'.

Related:

• Issue Add documentation/help about supported config types #597 - Original documentation request
• PR fix: reject non-YAML extensions for --sqlconfig flag #684 - Documents YAML-only requirement

Implementation Notes:

• Check file extension in configureViper() or SetFileName() function
• Allow .yaml, .yml, and no extension (for default ~/.sqlcmd/sqlconfig)
• Provide helpful error message pointing users to documentation