ENH: Add support for relaying emails via `boto3` for use cases where apps running in AWS use IAM policies to authenticate against SES

[devraj]

Is your feature request related to a problem? Please describe.
Whilst Amazon SES offers relaying emails via SMTP there are cases where the environment is configured to use IAM policies for authentication. This applies to services like S3 for file storage or SES for email relay.

In these use cases the application would simply instantiate the boto3 client as:

import boto3
ses_client = boto3.client("ses", regional_name="ap-southeast-2")

and use their send_email function to send the message without the need of authenticating.

Describe the solution you'd like
If it's amicable with the design of this project then I would like to be able to initialise EmailSender with preconfigure boto3 client no different to how we can pass a custom SMTP instance.

import boto3
ses_client = boto3.client("ses", regional_name="ap-southeast-2")

sender = EmailSender(boto3_client=ses_client)

the send function would then use the boto3 client instead of an SMTP server to relay the emails.This way we get the array of functionality (namely templating) that this library offers and integrate into a platform like AWS.

Describe alternatives you've considered
At the moment our only alternative is to render Jinja templates on our own and then relay messages via boto3.

Additional context
This may be relevant to other cloud services like GCP or Azure. We've been working with the lovely folk @resendlabs who maintain a Python idiomatic SDK and could see this as an enhancement.

I would personally be willing to work on the boto3 and resend integration and provide a pull request :)