deps: Add explicit bigdecimal dependency

satreixrequested to merge
satreix/cel-ruby:add-bigdecimal-dep into master

From ruby 3.3.0 release note:

Standard library updates RubyGems and Bundler warn if users do require the following gems without adding them to Gemfile or gemspec. This is because they will become the bundled gems in the future version of Ruby.

These libraries include bigdecimal.

Closes #15 (closed)

Edited by satreix

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