* refactored the models to be strongly typed with structs and mapstruct
to decompose the dynamic parts of the JMAP payloads
* externalized large JSON strings for tests into .json files under
testdata/
* added a couple of fantasy Graph groupware APIs to explore further
options
* added k6 scripts to test those graph/me/messages APIs, with a setup
program to set up users in LDAP, fill their IMAP inbox, activate them
in Stalwart, cleaning things up, etc...
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