Add E2E tests for aggregation, fix race condition

[DaveTryon]

The bulk of this change is adding an E2E test to exercise aggregation. It aggregates only 1 source SBOM, with the hope that we can later add a second case with more than 1 source SBOM. While adding this, a race condition was discovered--the call to ISbomConfigProvider.ClearCache was sometimes being called before the Task.Run calls in FileHasher.Run had completed, leading to an Exception being thrown. Shout-out to @sfoslund for providing a fix that was tightly scoped and didn't involve extensive plumbing changes to make FileHasher.Run--and it entire calling stack--fully async.

Some changes were needed to the FileHasher unit tests, since the test now needs to include execution of the code inside the Task.Run block. These changes fell into 2 categories:

1. Populate the files object before calling the FileHasher.Run (I put this into a helper method since needed this in multiple places).
2. Use deconstruction because the analyzer was complaining without it.

[DaveTryon]

/azp run

[github-actions]

This PR changes files in the API project. Does it change any of the API interfaces in any way? Please note that this includes the following types of changes:

• Changing the signature of an existing interface method
• Adding a new method to an existing interface
• Adding a required data member to a class that an existing interface method consumes

Because any of these changes can potentially break a downstream consumer with customized interface implementations, these changes need to be treated as breaking changes. Please do one of the following:

Option 1 - Publish this as a breaking change

1. Update the documentation to show the new functionality
2. Bump the major version in the next release
3. Be sure to highlight the breaking changes in the release notes

Option 2 - Refactor the changes to be non-breaking

1. Review this commit, which adds a new interface in a backward-compatible way
2. Refactor the change to follow this pattern so that existing interfaces are left completely intact
3. Bump the minor version in the next release

[DaveTryon]

/azp run