Improve support for storing host env externally

[carolynvs]

One feature that I have been working on is being able to store a tool's entire host environment, e.g. claims, credential sets, externally instead of on the local host.

I'm going to submit a few PRs around this and wanted to have a single issue to help give context them, as they really all came from a single branch to support this that I split up once I had it all working. 😀

• Improve crud.Store so it's easier to use the same instance for working with different types of data. As far as crud is concerned they are all []byte, though we do want to group them by type.
• Filesystem is easy and doesn't require connecting or releasing resources but most other data stores need our structs to account for this.
• Make CredentialStrategy generic so that we can lookup credential values from other locations than just the local host. For example, retrieving them from a key/value store like Hashicorp Vault.
• Other smaller related changes.

I'll do my best to make each independent but they may end up stepping on each other since they do affect the same files.

[carolynvs]

All the tasks from this issue have been implemented.