Feature Request: Conditional env validation (envEnableIf)

[arjunshajitech]

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:

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.

[arjunshajitech]

@caarlos0 Just a gentle ping for review