Alerting plugin supports sending html format email

[gaobinlong]

Relates to opensearch-project/notifications#586.

Some users asked if we can support sending html format alerting email, but html format email may introduce security issue so we implemented html sanitization in notification plugin which can sanitize the html email before sending it, the PR in notification plugin is opensearch-project/notifications#694, and there are still some work need to be done in alerting plugin to make the feature complete, the following shows the work:

1. common-utils plugin: need to add a bool parameter such as isHtmlFormatMessage to identify whether the email message is html format or not, we have to add the parameter into this plugin because most of the objects like monitor, triggers used by alerting plugin are defined in this plugin.

2. alerting plugin: create or update monitor API can accept the bool parameter isHtmlFormatMessage, when calling the sendNotification method to send email, if isHtmlFormatMessage is true, use htmlDescription parameter instead of textDescription.

[gaobinlong]

@lezzago can you help to assign this issue to me? I'll do the task.