Add Permalink to Msg struct

[jreinstra]

At least in the Slack list-stars method if not others, a permalink is included with each item returned. This permalink is not available via slack-go because it is not a field on the struct used to parse that response, so this change adds that missing field.

Pull Request Guidelines

These are recommendations for pull requests.
They are strictly guidelines to help manage expectations.

PR preparation

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

Should this be an issue instead

• is it a convenience method? (no new functionality, streamlines some use case)
• exposes a previously private type, const, method, etc.
• is it application specific (caching, retry logic, rate limiting, etc)
• is it performance related.

API changes

Since API changes have to be maintained they undergo a more detailed review and are more likely to require changes.

• no tests, if you're adding to the API include at least a single test of the happy case.
• If you can accomplish your goal without changing the API, then do so.
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• in library cache for users. caches are use case specific.
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[kanata2]

Thanks for your contribution! I commented, please confirm.

[jreinstra]

Thanks for your contribution! I commented, please confirm.

Oh whoops, accepted fix, thanks!

[kanata2]

I found one minor style-related fix, so please confirm 🙏

[jreinstra]

I found one minor style-related fix, so please confirm 🙏

whoops, accepted, thanks!

[kanata2]

LGTM! Thanks.