Fix error in Mapper::Normalizer when CI include value type is invalid

What does this MR do and why?

In #425139 (closed), we logged a backtrace that indicated an error:

undefined method `deep_symbolize_keys' for 123:Integer

This error occurs when expanding CI includes at Config::External::Mapper::Normalizer::29.

It can be reproduced by specifying an invalid value type for the include: keyword like so:

include: 123 # Also fails when this is a Boolean

It also errors when you include an array of locations like so:

include:
  - 123 # Also fails when this is a Boolean or Array

This MR fixes this problem by adding type validation on the location value. It should only be either a String or Hash type.

Notes about the approach:

• You might be wondering why this validation isn't already covered by validates :config, hash_or_string: true in Ci::Config::Entry::Include. This is because included files are expanded and mostly validated before @root.compose! runs in Ci::Config.
• Some of the validations in Ci::Config::Entry::Include do get executed, but that's only in certain scenarios where the include keyword "falls through" to the @root.compose! step. This can happen if it's not removed by Ci::Config::External::Processor#remove_include_keyword!. I think this is why we haven't just removed these Entry classes yet.
• For the reasons above, it seems the most appropriate to add the validation logic to Ci::Config::External::Mapper::Normalizer. Also, it:
  • Aligns with the notion of "normalizing"
  • Is one of the first steps in the file expansion process
  • Is directly dependent on the value type being a string or hash

Resolves #425139 (closed).

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Screenshots

Before fix	After fix

How to set up and validate locally

1. Update your project's .gitlab-ci.yml file with:

include: 123

2. Observe that it outputs a validation error message as seen under Screenshots.

Related to #425139 (closed)