validate sns message attributes and remove if invalid

[qinxx108]

Add validation for message attribute, if not a validate attribute, we will remove the attribute

Separate from https://github.com/prometheus/alertmanager/pull/2808

[alvinlin123]

@simonpasquier would you be able to kindly restart the failed workflow? It failed due to transient issue it seems like:

error pulling image configuration: Get https://quayio-production-s3.s3.amazonaws.com/sha256/57/57b64f8a7d6913f7c594d442d7d51bd1814d32b8717b75ef7d038c4f1d085015?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAI5LUAQGPZRPNKSJA%2F20220214%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20220214T090947Z&X-Amz-Expires=600&X-Amz-SignedHeaders=host&X-Amz-Signature=0e9aa0b2baf73e04e98ae98851876611eb2570a6bc5933bac380d624315dbad1: net/http: TLS handshake timeout
!! exit status 1
The base builder docker image exited unexpectedly

[qinxx108]

@gotjosh @simonpasquier Could you take a look at this as well tysm!