add ImmutableMetadata semantic type

[gregcaporaso]

This would allow actions to output metadata, which is currently not possible. This type would be viewable as qiime2.Metadata, so could be used anywhere that tsv (i.e., mutable) metadata can be used, and if a user wants to edit their ImmutableMetadata, they will simply export it at which point it becomes a plain, old QIIME 2 metadata tsv file.

This would be very helpful for actions like qiime feature-table group (qiime2/q2-feature-table#194), where the sample identifiers change. Actions like this would have control over how input metadata is transformed into ImmutableMetadata.