Migrate from FluentAssertions

[WhitWaldo]

Despite being Apache licensed for a long time, the author of the FluentAssertions library has elected to make a shift to a commercial license starting with v8

We're currently using version 5 of FluentAssertions across this SDK, but it's time to migrate to another tool.

Several users on that thread suggested switching to Shouldly and it's my understanding that our Apache license is compatible with the 3-clause BSD license they use. The syntax looks similar and should be a relatively easy migration.

Another alternative is switching to a fork of FluentAssertions before the license change, now hosted at AwesomeAssertions. This would mean we don't have to make any immediate code changes, but the README does state that one shouldn't expect updates to the fork, so we'd potentially be putting off a migration to another project in the future anyway.

I have a completed PR here to migrate to Shouldly, but wanted to open up the floor to any other suggestions.

[jcfnomada]

Please consider again https://github.com/AwesomeAssertions/AwesomeAssertions. There are a lot of old a recent contributers already in the project, so it is expected to continue it's work with bug fixes and new features.

[WhitWaldo]

Please consider again https://github.com/AwesomeAssertions/AwesomeAssertions. There are a lot of old a recent contributers already in the project, so it is expected to continue it's work with bug fixes and new features.

I'll certainly keep an eye on what they're doing over here. The Shouldly PR is a hedge against that not working out.