Add en-GB ID Numbers (aka National Insurance Numbers)

[stefanjcollier]

Motivation / Background

In GB, we use national insurance numbers as our primary citizen identification numbers.

Additional information

National Insurance Numbers are 9 digit strings that are typically 2 alpha characters followed by 6 numbers and finally a single alpha. There are few combinations are not permitted as seen here.

The valid regex provided will produce a subset of all legal NI numbers. I didn't consider a regex to produce all combinations wasn't necessary.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug, refactor something, or add a feature.
• Tests and Rubocop are passing before submitting your proposed changes.

If you're proposing a new generator or locale:

• Double-check the existing generators documentation to make sure the new generator you want to add doesn't already exist.
• You've reviewed and followed the Contributing guidelines.

[stefanjcollier]

Faker::IdNumber.valid

Faker::IdNumber.invalid

[stefannibrasil]

Thank you for the screenshots! What do you think of adding a note in the documentation for this locale that the result will be normalized? The doc could have a section for locales: https://github.com/faker-ruby/faker/blob/main/doc/default/id_number.md?plain=1

[stefanjcollier]

Great shout! I've attempted to document it in the same style and writing style.
I hope this was how you were intending?

Happy to rewrite according to any examples. Feedback very welcome, I doubt you could offend me. 😄

[stefannibrasil]

Thank you @stefanjcollier I left some suggestions.

[stefannibrasil]

Suggested change

	Besides the locale-specific ID Number methods. Faker supports retrieving localised calls to `.valid` and `.invalid`.
	Here is an example:
	Besides the default ID Number methods, Faker supports localized `.valid` and `.invalid` values. Here is an example:

[stefannibrasil]

Suggested change

	### en-GB
	When provided with British English, unformatted'[National Insurance](https://www.gov.uk/national-insurance/your-national-insurance-number)' numbers are generated.
	Note: Faker can only generate a subset of all possible legal/illegal national insurance numbers.
	```ruby
	Faker::Config.locale = 'en-GB'
	Faker::IdNumber.valid #=> "AJ405924A"
	Faker::IdNumber.invalid #=> "BG316764W"
	```
	### en-GB
	When the locale is set to British English, unformatted'[National Insurance](https://www.gov.uk/national-insurance/your-national-insurance-number)' numbers are generated:
	```ruby
	Faker::Config.locale = 'en-GB'
	Faker::IdNumber.valid #=> "AJ405924A"
	Faker::IdNumber.invalid #=> "BG316764W"

Note: Faker generates a subset of all possible legal/illegal national insurance numbers.