itest: export HashmailHarness struct

[ViktorT-11]

This PR exports the HashmailHarness struct, so that it can be used as an external dependency to setup a local mail server.

[ellemouton]

looks like we should maybe update the linter config. Linters like exhaustivestruct have been deprecated :)

[ViktorT-11]

looks like we should maybe update the linter config. Linters like exhaustivestruct have been deprecated :)

Ok thanks, was a bit confused about what's happening with the CI! I will look into that @ellemouton!

[ViktorT-11]

Ok finally got the linter to work properly :)! I updated the config to use a similar config to LND, as I figured it's likely our most up to date linter configs.

I pushed the update linter settings as 2 separate commits to this PR. Let me know if you think it's better that I push them as a separate PR instead @ellemouton!

[ellemouton]

LGTM :) one nit :)

[ViktorT-11]

Thanks for the review and the feedback @ellemouton! Addressed it with the latest push :)

[lightninglabs-deploy]

@bitromortac: review reminder

[bitromortac]

LGTM 👍, sorry for the late review

[ViktorT-11]

Thanks for the review @bitromortac!

[ellemouton]

@ViktorTigerstrom - the LNC repo prevents me from merging if the commits are not signed (see the "Merging is blocked" warning above)

[ViktorT-11]

Updated the commits' author to my GPG key email address & signed the commits.