Add string_number_fields for tests

[jsoriano]

What does this PR do?

Add a new option similar to numeric_keyword_fields, that allows to indicate fields with numeric types that can accept string values if they can be parsed as numbers.

Find implementation in https://github.com/elastic/elastic-package/pull/2065.

Why is it important?

This is needed to introduce validation on fields that were not being tested, without needing to modify the existing functionality (https://github.com/elastic/elastic-package/pull/2054).

Checklist

• [x] I have added test packages to test/packages that prove my change is effective.
• [x] I have added an entry in spec/changelog.yml.

Related issues

• Relates to https://github.com/elastic/elastic-package/pull/2065.
• Relates to https://github.com/elastic/elastic-package/pull/2054.

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: ea65d05dba1ebc51e8c207749db24069eed669b0

cc @jsoriano