Add RBS support for Ruby 3+

[splittingred]

What? Why?

Adds .rbs files for type checking support for Ruby 3+.

Note we're not implementing Steep or other static checkers yet as libraries Gruf depends on are not yet fully RBS-supported.

How was it tested?

rspec, rubocop, and through an IDE (such as RubyMine) that has native RBS support.

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@bigcommerce/ruby

[splittingred]

Holding on this for a bit until we're sure we want to adopt RBS as-is.