Add the ability to provide own scheduled executor for SMTP timeout

[jmehrens]

Add the ability to provide own scheduled executor for SMTP write timeout. This will allow sharing of an external thread pool for all SMTP connections.

Details and original author credits our by @egor-ponomarev in PR #22

[jmehrens]

Committed as
bcace35
9d0cc38

[jmehrens]

Article on the feature by original author:

https://medium.com/@egorponomarev/my-contribution-to-javamail-and-sending-emails-using-spring-boot-3-a03f814e899c