Support type field map during pass through env variable #103

Problem

Confita doesn't support map types when loading from environment variables:

type Config struct {
    Settings map[string][]int `config:"settings"`
}

Current error

Error: failed to parse config: field type 'map' not supported

Proposal

Add support for maps by parsing JSON from env variables:

export SETTINGS='{"key1":[1,2,3],"key2":[4,5,6]}'
export ACT_RECEIPT_UZ_ALLOWED_INVOICE_PAYMENT_METHODS_BY_SERVICES='{7: [1101], 37: [1101], 671: [1101]}'

Benefits

• More flexible configuration options
• Support for complex nested structures
• Consistency with other config libraries (viper, envconfig)