Add equals marker and extend existing validators for cross-field validation

[sivchari]

Description

Implement cross-field validation by adding an equals marker and extending existing comparison operators to reference other fields.

Implementation Details

1. New equals marker:

type PasswordForm struct {
    // +govalid:minlength=8
    Password string
    // +govalid:equals=Password
    Confirm  string
}

2. Extend existing comparison operators:

type PriceRange struct {
    // +govalid:required
    MinPrice int
    // +govalid:gt=MinPrice  // References MinPrice field
    MaxPrice int
}

type DateRange struct {
    StartDate time.Time
    // +govalid:gte=StartDate  // References StartDate field
    EndDate   time.Time
}

Parameter interpretation:

• Numeric values: Compare as fixed values (current behavior)
• Identifiers: Reference fields in the same struct
• equals marker supports all types for equality checking

Success Criteria

• equals marker implemented
• Existing operators support field references
• Clear distinction between values and field references
• Comprehensive test coverage

[sivchari]

In addtion, notequals, too.