RPM Support

In libmodulemd, it is planned to introduce an extension to version 2 of modulemd documents which differ slightly from the current v2. A new, optional field is added, called "static_context", described thusly:

    # context:
    # Module context flag
    # The context flag serves to distinguish module builds with the
    # same name, stream and version and plays an important role in
    # automatic module stream name expansion.
    #
    # If 'static_context' is unset or equal to FALSE:
    #   Filled in by the buildsystem.  A short hash of the module's name,
    #   stream, version and its expanded runtime dependencies. The exact
    #   mechanism for generating the hash is unspecified.
    #
    #   Type: AUTOMATIC
    #
    #   Mandatory for module metadata in a yum/dnf repository.
    #
    # If 'static_context' is set to True:
    #   The context flag is a string of up to thirteen [a-zA-Z0-9_] characters
    #   representing a build and runtime configuration for this stream. This
    #   string is arbitrary but must be unique in this module stream.
    #
    #   Type: MANDATORY
    static_context: false
    context: c0ffee43

Source: https://github.com/fedora-modularity/libmodulemd/commit/2000e88d48a7b8d9fba1f7866d3709035dd2b957#diff-f693f5c1bd57e7782999ed7d59a4e2cdb1548c6d5e2e4663265f2165a949effeR81-R96

(see that commit also for document examples, and the names of libmodulmd functions)

See also: https://github.com/fedora-modularity/libmodulemd/commit/0132015b5729b6077d49fc5beb2c662a563c6e6d#diff-f693f5c1bd57e7782999ed7d59a4e2cdb1548c6d5e2e4663265f2165a949effe

We need to add the model fields/migration and code necessary to support reading and writing this metadata (sync, upload and publish).