Add support for multi-file NPE encrypt and decrypt

[b-long]

For performance, this PR adds several new functions:

func EncryptFilesInDirNPE()
func EncryptFilesGlobNPE()
func DecryptFilesInDirNPE()
func DecryptFilesGlobNPE()

This will allow Python clients to perform bulk work, with a single translation to/from python-to-golang. In addition, we'll allow the caller to reuse the sdkClient during the bulk operations.