Gets a tag by the specified slug.
The getTagBySlug() function returns a Promise that resolves to a tag whose slug matches the specified slug.
The slug is the end of a tag's URL that refers to a specific tag. For example, if a tag's URL is https://example.com/blog/tag/{my-tag-slug}, the slug is my-tag-slug. The slug is case-sensitive and derived from the tag's label.
Slug of the tag to retrieve.
Options specifying which additional fields to return.