Introduce String converter for Gmail connector

(Migrated from https://github.com/syndesisio/syndesis/issues/3517)

Author: Keith Babo
Assignees: Andrea Cosentino,

This is a...

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[X] Feature request
[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report [!-- Please search GitHub for a similar issue or PR before submitting --]
[ ] Documentation issue or request
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The problem

It would be nice if the Gmail connector included a type converter for String -> GmailMessageModel. This would allow non-Gmail-aware components/processors to work with Gmail connector OOTB by just setting a String as the message body. The converter should set the String body value to GmailSendEmailCustomizer.text.

Expected behavior

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Pass a string to the Gmail connector and have it used as the email text.