adds functionality to add custom operations

[itsthatguy]

This feature was requested here: #40

I'm not sure how you would want to approach this. Open to feedback.

Summary of Changes

Runtime Custom Operator Registration

• Added ability to register custom operations at runtime (didn't exist
  before)
• Three new operator types: regular, lazy, and data operations
• Custom operators can override built-in operators

New Public API Functions

// None of these existed before this PR
pub fn add_operation(name: &str, operator: OperatorFn, num_params:
NumParams) -> Result<(), Error>
pub fn add_lazy_operation(name: &str, operator: LazyOperatorFn, num_params:
NumParams) -> Result<(), Error>
pub fn add_data_operation(name: &str, operator: DataOperatorFn, num_params:
NumParams) -> Result<(), Error>
pub fn remove_operation(name: &str) -> Result<bool, Error>
pub fn clear_operations() -> Result<(), Error>

New Error Types

• Added Error::RegistryError variant for custom operator registry failures

New Exports

• Made Error type public (pub use error::Error)
• Exported operator function types: OperatorFn, LazyOperatorFn,
  DataOperatorFn, NumParams

New Dependencies

• Added once_cell = "1.19" dependency

Internal Architecture Changes

• Added dynamic operator registry system with thread-safe concurrent access
• Extended existing operator system to support runtime registration
• Added CustomOperatorRegistry with RwLock<HashMap> for each operator type

What Remained Unchanged

• Core apply() function - same signature and behavior
• All existing built-in operators work exactly the same
• No breaking changes to existing API
• All existing functionality preserved