Alec Baldwin Considering Calling Celeb Witnesses in Manslaughter Trial

Alec Baldwin may look to enlist the help of other celebs when his manslaughter trial over Halyna Hutchins' death gears up this summer -- but there's a catch ... TMZ has learned. Sources with knowledge of the case tell TMZ ... the actor's team is considering putting together a list of famous people to potentially testify on Alec's behalf during his trial in July -- but we're told these stars would be people who've…