Declare dependency on base64

[Earlopain]

Ruby 3.3 which will be released pretty soon has added a warning for using base64 without declaring it since it will be removed as a default gem in Ruby 3.4. See https://github.com/ruby/ruby/blob/fe9e57bac8ec73ad822c1b245376a6cd96139ccb/NEWS.md#stdlib-updates for more details.

Currently there will be no warning for this gem I believe because it depends on both activesupport and faraday which declare base64 as their dependency. Faraday will likely remove this dependency with lostisland/faraday#1541, leaving only activesupport. For future compatibility, should both remove their reliance on the base64 gem, it should be declared here as well.

[nov]

OK, I prefer this approach than lostisland/faraday#1541, cedarcode/webauthn-ruby#402 etc.