Feature Request: Conditional env validation (envEnableIf)

Currently it is not possible to conditionally require environment variables based on another environment variable.

A feature like `envEnableIf` would be very helpful for cases where some configuration is only needed when a feature is enabled.

For example, assume a server supports email sending:

```go
type Config struct {
EmailEnabled bool `env:"EMAIL_ENABLED"`
SMTPHost string `env:"SMTP_HOST" envEnableIf:"EMAIL_ENABLED"`
SMTPPort int `env:"SMTP_PORT" envEnableIf:"EMAIL_ENABLED"`
}
```

Expected behavior:

* If `EMAIL_ENABLED=false`, SMTP variables are ignored.
* If `EMAIL_ENABLED=true`, `SMTP_HOST`, `SMTP_PORT`, etc. should be required and validated.

This helps validate related configuration only when the feature is enabled, instead of requiring those variables all the time.