File-scoped namespaces and other improvements

[Banovvv]

File-scoped namespaces and other improvements

• Fixed += string mutation
• Fixed HashHMAC to dispose properly
• Used StringBuilder in CreateSignedUrl to avoid repeated string allocations
• Added AddCloudConvertAPI() extension method with DI support
• Updated README.md

[josiasmontag]

Thanks for your contribution!
Unfortunately, it is hard to review this PR because it changes the indentation in all files.
Can you either undo this or alternatively split up the PR so that we can see the actual changes?

[Banovvv]

@josiasmontag the indentation is a side-effect of the file-scoped namespaces. I'll do what you suggest and create a second PR only with the file-scoped namespaces.