Add base64 as runtime dependency

```
$ ruby --version
ruby 3.3.4 (2024-07-09 revision be1089c8ec) [arm64-darwin23]

$ bundle exec rspec
/app/vendor/bundle/ruby/3.3.0/gems/specinfra-2.90.0/lib/specinfra/backend/powershell/script_helper.rb:1: warning: base64 was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec. Also contact author of specinfra-2.90.0 to add base64 into its gemspec.
```

c.f. https://www.ruby-lang.org/en/news/2023/12/25/ruby-3-3-0-released/

specinfra.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |spec|
   # TODO: at some point pin to a minumum version of ruby to reduce support burden in a major version bump
   # spec.required_ruby_version  = '>= 2.3.0'
 
+  spec.add_runtime_dependency "base64"
   spec.add_runtime_dependency "net-scp"
   spec.add_runtime_dependency "net-ssh", ">= 2.7"
   spec.add_runtime_dependency "net-telnet" # intentionally version-unspecified for Ruby older than 2.3.0