Add frozen_string_literal to CompositeIO and Multipart

[allan-pires]

Description

This adds the magic comment for making string literals frozen on CompositeIO and Multipart.
We also utilize the unfrozen string +'' on a few vars to allow the rest of the file to utilize frozen string literals.

Context

There was a deprecation warning for chilled strings merged recently in Ruby: ruby/ruby@12be40a

And with that started to see the following warnings on google-apis-core:

 /vendor/gems/3.4.0/ruby/3.4.0+0/gems/google-apis-core-0.11.3/
 lib/google/apis/core/composite_io.rb:45:in 'String#replace':

/vendor/gems/3.4.0/ruby/3.4.0+0/gems/google-apis-core-0.11.3/
lib/google/apis/core/composite_io.rb:52:in 'Google::Apis::Core::CompositeIO#read':

/vendor/gems/3.4.0/ruby/3.4.0+0/gems/google-apis-core-0.11.3/
lib/google/apis/core/multipart.rb:63:in 'Google::Apis::Core::FilePart#to_io':

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