feat: Added GST validations to Sales Invoice

[vorasmit]

Summary of Changes

• Improved Get GST Accounts --> based on PR refactor!: Clean GST accounts table and patches #6
• Improved existing validation of Sales Invoice Number
• Added GST Settings where users can opt out if they are unregistered.
• Validate same items with different Item Tax Template
• Validate GST Accounts in Sales Invoice.
• Validate if mandatory fields are present.
• Fixed HSN Validation

GST Settings

Check where taxes are not applicable

Check where incorrect tax accounts could be used

Mandatory Field Validation

TODO: In later PRs

• Update existing get_gst_accounts with new function. Update existing get_gst_accounts with new function.  #22