add compression options for write_file_md5()

[jmbarbone]

can take some inspiration from readr::write_rds():

compress <- match.arg(compress, c("none", "gz", "bz", "xz"))
con <- switch(
  compress,
  none = file(file, ...),
  gz = gzfile(file, ...),
  bz = bzfile(file, ...),
  xz = xzfile(file, ...)
)

write_function(x, con, ...)

all functions need to be rewritten to accept connections and set encoding via that function instead.