Add some error type convenience functions for mocking and inspection

[jburnham]

Description

Provide the ability to create the RequestError, etc types in other packages with the underlying
Error type. Useful in our code for mocking these error types.

Add the InternalErrorCodeIs to these errors as well to simplify checking the internal codes when
working with these types.

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[github-actions]

changelog detected ✅

[jacobbednarz]

once you add a CHANGELOG, i'm 👍 to this one.

[codecov-commenter]

Codecov Report

Merging #1047 (fa904bd) into master (6c5ea4a) will increase coverage by 0.95%.
The diff coverage is 64.73%.

@@            Coverage Diff             @@
##           master    #1047      +/-   ##
==========================================
+ Coverage   49.06%   50.01%   +0.95%     
==========================================
  Files         108      115       +7     
  Lines       10428    11040     +612     
==========================================
+ Hits         5116     5522     +406     
- Misses       4200     4347     +147     
- Partials     1112     1171      +59     

Impacted Files	Coverage Δ
access_application.go	76.52% <ø> (ø)
access_bookmark.go	72.44% <ø> (ø)
workers_account_settings.go	35.71% <35.71%> (ø)
rulesets.go	35.09% <47.36%> (+7.43%)	⬆️
workers_tail.go	52.00% <52.00%> (ø)
workers_subdomain.go	57.14% <57.14%> (ø)
email_routing_settings.go	60.00% <60.00%> (ø)
email_routing_rules.go	65.64% <65.64%> (ø)
lockdown.go	57.69% <66.00%> (+6.94%)	⬆️
email_routing_destination.go	66.25% <66.25%> (ø)
... and 14 more

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[jburnham]

once you add a CHANGELOG, i'm 👍 to this one.

added

[github-actions]

This functionality has been released in v0.48.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!