Create reusable mixin node type to hold the SHA-1 hash

For MODE-31 and MODE-680, the sequencers for Teiid VDBs and XMI models are capturing the SHA-1 hash of the files, and including those in the sequenced output. Although the node types defined by the Teiid sequencers could actually define their own properties, it does seem useful to have a reusable mixin type that would define a single optional property to hold the SHA-1 hash. Although the SHA-1 (and other secure hashes) are actually binary, storing them in hexadecimal form make them readable and much more easily queryable. (Both Git and Maven use hexadecimal representations of SHA-1 hashes.)