refactor!: modernize base types and interfaces (#87) · oras-project/oras-dotnet@6adeaaa

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L832

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L882

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L882

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L891

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L526

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L530

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L532

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L535

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L629

Dereference of a possibly null reference.

Run unit tests: src/OrasProject.Oras/Remote/Repository.cs#L629

Converting null literal or possible null value to non-nullable type.

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