fix: refactor email sending functions

[J0]

What kind of change does this PR introduce?

The overall goal of this to expose a unified interface for emails. So that we can potentially implement the Hook as a Custom Mailer. This is to ensure that the impact on the existing code flow is minimal and that we can turn off the Hook easily if needed.

After this change, we can do something similar to:

mailer. := a.Mailer()
if a.config.Hook.Enabled {
    mailer =   a.CustomMailer
} 

and have all Hook logic live in the custom Mailer

Specific changes are:

• Removes context from mailer as it is currently unused
• pushes down mailer into respective email sending methods
• Adds remaining send methods as API methods
• Fetch OTP Length and MaxFrequency from config
• Add convenience function for checking if an email was sent within frequency limit
• push down externalURL and referrer into send function