Support creating multiple tags

[n7studios]

Summary

Adds a create_tags() method to support creating multiple tags in a single API request, as reported in #60

Testing

• testCreateTags: Tests that creating multiple tags works as expected.
• testCreateTagsBlank: Tests that creating multiple blank tags returns a ClientException
• testCreateTagsThatExist: Tests that creating multiple tags that exist returns a ClientException

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)