TG-910: feat: custom email template for alerts (#2400)

* Issue #added custom subject feature

* Issue #corrected the indentation

* Issue #added the custom html file

* Issue #added the template config

* Issue #corrected the template error

* Issue #corrected the template error

* Issue # removed the slakc title link

* Issue # added the api response latency alert rules.

* Issue # added the api response latency alert rules.

* Issue # added the api response latency alert rules.

* Issue # added the api response latency alert rules.

* Issue # added the api response variable

* Issue # TG-910 corrected the data type error

* Issue # TG-910 corrected the slack title

* Issue # TG-910 removed the unwanted templates

* Issue # TG-910 removed the nodename and instance name from template

* Issue # TG-910 updated the var

* Issue # TG-910 updated ansible var

* Issue # TG-910 added the description for vars.