Change `key` accessing for default `ContextOperator`

[kozmod]

Description

Default ContextOperator uses genetic type any(v0.2.3) (comparable #88 ) as key for storing/getting Tx.

Changing the key field to the function that return (calculate) the key, will make ContextOperator more flexible.

// ContextOperator inject and extract Tx from context.Context.
type ContextOperator[B comparable, T Tx] struct {
	key func() B
}

// NewContextOperator returns new ContextOperator.
func NewContextOperator[B comparable, T Tx](key func() B) *ContextOperator[B, T] {
	return &ContextOperator[B, T]{
		key: key,
	}
}

// Inject returns new context.Context contains Tx as value.
func (p *ContextOperator[B, T]) Inject(ctx context.Context, tx T) context.Context {
	return context.WithValue(ctx, p.key(), tx)
}

// Extract returns Tx extracted from context.Context.
func (p *ContextOperator[B, T]) Extract(ctx context.Context) (T, bool) {
	c, ok := ctx.Value(p.key()).(T)
	return c, ok
}