Use model.Duration for timeout option in webhook notifier

[alexander-akhmetov]

Changed webhook timeout field from time.Duration to model.Duration to make JSON marshalling consistent with YAML.

time.Duration marshals to JSON as an integer (nanoseconds), while to YAML (with gopkg.in/yaml.v3) it's marshalled as a duration string.

time.Duration example: https://go.dev/play/p/Ml2Mb_VFNyu:

JSON: {"timeout":90000000000}
YAML: timeout: 1m30s

model.Duration also marshals to human-readable duration strings in both JSON and YAML, and is consistent with other duration fields in Alertmanager.

model.Duration example: https://go.dev/play/p/rqts9yDSEQL

JSON: {"timeout":"1m30s"}
YAML: timeout: 1m30s