Related to #782 and #444
- Allows concurrent decryption of different secrets files
- Caches decrypted secrets by original file path and returns decrypted results from memory
- Secrets being run through an instance of helmexec will be cached and run as fast as possible concurrently
NB: This particular PR doesn't make _all_ calls to secrets cached and concurrent. Environment Secrets in particular seem to not be evaluated with a ScatterGather(), and doesn't use the same helmexec instance as other parts of the code, so it doesn't take advantage of these changes. Some reworking of the plumbing there would be needed.