Support JWT auth for HTTP notifier

[onimsha]

Currently http notifier does not support Token authentication using JWT, i'd like to add support to it as following:

config prefix: consul-alerts/config/notifiers/http-endpoint/

Key | Type | Description token | base64-encoded | Token to set on the 'Authorization' header

Key	Type	Description
token	base64-encoded	Token to set on the 'Authorization' header

When sending payload, http notifier will use the token and create the header as following 'Authorization: Bearer $token' .

[fusiondog]

I assume that a separate process would be required to set the token and consul-alerts would just be in charge of presenting whatever would be found there.

[onimsha]

Yes, since the notifier config is using consul KV, we can sett the token in the config prefix.