replace base64enc package with other package (openssl/jsonlite)

[m-muecke]

Removing the base64enc package by using openssl or jsonlite would remove another dependency that's already used in httr2, as well as one that is better maintained by Jeroen.
Usage here: https://github.com/r-lib/httr2/blob/c7d4a865a6e94a9f482a11044dd066c7fd7ac14d/R/utils.R#L123

[zacdav-db]

I'm happy for you to make a PR of what that may look like and then I can review.