Major refactoring to make slsm simpler and easier to work with

[aarongable]

This major refactoring -- nearly a rewrite -- of sunlight-secretmanager removes unnecessary layers of complexity, streamlines tests, and makes the project easier to understand and evolve moving forward. It also updates sunlight-secretmanager to target v0.5.0 of Sunlight, which has made several changes to the config format since this project was started.

Specifically:

• Move all the code directly into //cmd/sunlight-secretmanager, removing the "config" and "secrets" packages, which did not have coherent public interfaces or private implementation details worth protecting.
• Remove the "nameSeedMap" and "fileNamesMap" concepts, and instead let main directly introspect the loaded config.
• Remove the "Secrets" and "Filesystem" types, which were unnecessary layers of indirection around the AWS API and an integer, and which made testing more complex.
• Remove the modular "verifyFilesystem" func, and replace it with simply an integer indicating what type of filesystem should be verified.

This change will be followed by a feature change which adds the ability for sunlight-secretmanager to generate new seeds when it finds a configured log that does not yet have a seed stored in AWS.

[mcpherrinm]

By default, the SDK checks the AWS_PROFILE environment variable to determine which profile to use

I know you're not touching this line, but I'm pretty sure this WithSharedConfigProfile line isn't needed

[aarongable]

Merging to unblock the feature work; we can always come back and improve things more in the future.