Treat devicemapper thin pool feature args more formally

[mulkieran]

In stratisd we both read and set these args and we store a select few in the Stratis metadata.

They are really a distinct set of keys, but they are represented as Strings. We should have a more enum-based way to represent them, either in devicemapper or in stratisd, in order to avoid the generic set of problems that arise from using undifferentiated strings for keys.

[mulkieran]

This has been looked into stratis-storage/stratisd#3583. The most useful bit from that was the enum crate.