httr -> httr2

[sckott]

• vcr on cran only has support for httr, not httr2. httr2 support is in the works but not ready quite yet.
• rcromwell uses vcr.
• which means we need to use httr in rcromwell and this pkg for now

move to httr2 once vcr has support for it on github at least (& cran before rcromwell goes to cran)