Use immutable strings in our data model

[blt]

We know from #6831 that vector has high allocation pressure. It's basic datum -- the Event -- encodes a String in several places but does not mutate this string. We can shave at least one word from each Event by using Box<str> in these places. Resolution of this ticket will collide somewhat with #5374 but the work is complementary. This will also have bearing on #6673 as well, buying headroom if nothing else.

[blt]

Had some work put into this project here #6846. Ultimately closed for lack of evidence that this would effect vector performance today.

[blt]

I'm closing this. I do not believe we can tackle this as a singular project, more a long-term goal that we'll pick at in reviews.