[nuget-archive] Add index integrity attribute to nuget_archive

[dpvdberg]

Allow explicit integrity for index.json in the nuget_archive repo rule.
Solve the following issue: #431

I previously created #432, but in that PR I committed with a different e-mail for which I did not sign a CLA.

[dpvdberg]

I can't seem to solve the docs build issue, could I maybe get some help with this?
When I run bazel run //docs:update, it appears my platform is incompatible?

~\...\rules_dotnet>bazel run //docs:update
Target //docs:update failed to build
Use --verbose_failures to see the command lines of failed build steps.
ERROR: Analysis of target '//docs:update' failed; build aborted: Target //docs:update is incompatible and cannot be built, but was explicitly requested.
Dependency chain:
    //docs:update (261f83)
    //docs:update_0 (261f83)
    //docs:defs-docgen.md (261f83)
    //docs:defs (261f83)   <-- target platform (@@local_config_platform//:host) didn't satisfy constraint @@platforms//:incompatible
INFO: Elapsed time: 0.274s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target