Support team.profile.get method

[MattDavisRV]

This PR is a duplicate of #474 that was closed due to no activity on my part. Reopening to have it merged as I believe others could benefit from it.

This PR adds support to retrieve custom fields defined on the Team, which can be populated via the users.profile.set endpoint. Profile field variables are custom to the team, so being able to retrieve them is important when setting new values. These values can also be found within the profile section of the Admin Console as noted in the comment of the function.

This PR also adds support for updating the custom fields on a user using these variables. The new function accepts a map of the existing struct UserProfileCustomField. The key of the map is the custom Team Variable to define the custom field on the user.

Note that any existing field set, but not passed to function to be updated will be cleared based on Slack's native functionality.

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Run make pr-prep from the root of the repository to run formatting, linting and tests.

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[MattDavisRV]

I'm not sure why the lint test is failing, I did run make pr-prep prior to putting this in and everything passed.

[kanata2]

The failure of linter check is caused by golangci-lint's upgrade, so don't worry about this 🙏