feat: Add conditional validation with if(cond = <expr>, <rules>...) syntax

[aobatact]

close #78

This PR introduces conditional validation functionality to garde, allowing users to apply validation rules only when specific conditions are met. This enables more flexible and dynamic validation logic based on field values or context.

Syntax:

#[garde(if(cond = <expression>, <rules>...))]

Alternative Syntaxes Considered

Option 1: Global scope modifier

#[garde(ascii, length(min = 3), if(self.enabled))]
This creates ambiguity about which rules the condition applies to and breaks the independence of individual validation rules.

Option 2: Per-rule conditions

#[garde(ascii(if = self.check_ascii), length(min = 3, if = self.check_length))]
While this maintains rule independence, it becomes verbose, and less efficient when multiple rules share the same condition.

Option 3: Simple wrapper syntax

#[garde(if(self.enabled, ascii, length(min = 3)))]
While simpler, this creates order dependency between the condition and rules.