cleanup of newman package

[golanglemonade]

• fixes buildkite pipeline
• moves options and NewEmailMessageWithOptions to newman so it aligns with the other functions instead of being in shared
• Updates all providers New* functions to return the newman.EmailSender interface, rather than the explicit type so everything conforms to the interface.
• Adds a Send + SendWithContext so all providers have the same functions, aligns to the interface (in a few the context isn't used but would rather the interface is enforced and we can come back to ensuring it is used at a later time)
• Removes the stutter from function names, e.g. instead of resend.NewResendEmailSender, it is now just resend.New or instead of resend.ResendOption, it is now resend.Option
• Adds test for resend since this is our first-class provider
• Updates postmark to use httpsling instead of creating a http.Client from scratch
• Adds a mock provider and a WithDevMode option for the resend provider that will log emails to a directory instead of sending them when it is enabled

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
58.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud