Added support for the WEBSERVICE function

[paulkned]

This is:

- [ ] a bugfix
- [ x ] a new feature

Checklist:

• [ x ] Changes are covered by unit tests
• [ x ] Code style is respected
• [ x ] Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• [ x ] CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

The WEBSERVICE function was not supported, and now it is. The only thing I'm unsure of are the unit tests. Obviously this function needs to contact an external website. I chose a generic API which seems like it will stay in the air for a while. However, there is no way of knowing this for sure, neither is there any way of knowing if the API will keep returning the same results. This will break the unit tests without any code changes. But not including this test will make sure the function is not properly tested ... dilemma's, dilemma's.

[PowerKiKi]

What about using something like guzzle ? We can then assume guzzle is tested and working as expected and we may be able to mock it for our own tests ?

[paulkned]

@PowerKiKi Good point. I didn't want to add packages willy-nilly but with your blessing I added guzzle. This made mocking and testing a bit robuster.

[PowerKiKi]

This should live in \PhpOffice\PhpSpreadsheet\Settings, like the similar setCache/getCache, so settings are centralized and could maybe later be transformed into a PSR-11.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
If this is still an issue for you, please try to help by debugging it further and sharing your results.
Thank you for your contributions.

[patrickbrouwers]

@PowerKiKi @paulkned @MarkBaker is Guzzle 7 a hard requirement for this to work or could we loosen up the requirement to ^6.3.1|^7.0?

Installing in a clean Laravel 7.0 install currently doesn't allow 1.14 to be installed, which I'm afraid will cause a lot of confusing among the Laravel Excel users who want to upgrade to use the backwards compatible fix for the empty enclosures which was fixed in 1.14.

[mfn]

See also #1562

I did see the new release but wondering why it didn't upgrade => because my project uses Guzzle ^6