License checker and license update

[Bez625]

Hi @mcdee

I wanted to get your thoughts on this. Essentially 2 changes:

• Add a github action that ensures the expected license is present on all go files (the license is defined in .licenserc.json).
• A golang script that will use the template from .licenserc.json and update all go files to match.

Perhaps the second part is more of a generic tool that we want to store somewhere else and use as a binary across multiple repos, but I thought I'd open the change and get your feedback first.

I did run the updater as part of the PR and it's created a lot of changes, but I'm hoping with the github action enforcing new changes then we would only have to run this once per year to update to the current year.

[mcdee]

Regarding a license check, there is a goheader linter that can be enabled in .golangci-lint.yml, e.g.:

linters-settings:
  goheader:
    values:
      regexp:
        YEARS: '(20\d\d|20\d\d, 20\d\d|20\d\d - 20\d\d)'
    template: |-
      Copyright © {{ YEARS }} Attestant Limited.
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

I'm also not sure about the update process, because it resets the copyright date to the current year. Copyright starts with the date of first implementation, with (optionally) additional years added when there are major updates. I think that as long as we have the linter in place the update isn't necessary.

[Bez625]

That's a far cleaner solution - I'll look at getting it in place.