[PORTABILITY] Installable plugins

[blythed]

We would like to not be restricted to pre-installed code in the Dockerfile/ environment. Ideas:

• Add code to metadata or artifact_store
• Download code when program starts into ~/.superduperdb/plugins
• Use _object (exists already)
• 3rd party functionality

@jieguangzhou to provide additional ideas.

[blythed]

After design discussion with @jieguangzhou and @kartik4949 we have decided on something like this:

class Plugin:
    _artifacts = [('cache_path', file)]
    path: '<some_local_path>'
    cache_path: t.Optional[str] = ''
    install_on_load: bool = False

    def __post_init__(self):
        if not self.cache_path:
            self.cache_path = '~/.superduperdb/plugins'

    def on_create(self, db):
        db.artifact_store.put_file(self.path)

    @ensure_initialized
    def install(self):
        os.system('pip install -r ~/.superduperdb/plugins/{self.identifier}')

    def on_load(self, db):
        sys.path.append('~/.superduperdb/plugins')
        if self.install_on_load:
            self.install()

On building connection, potentially load plugins:

def build_datalayer(...):
    ...  # existing things
    if CFG.load_plugins:
        for plugin in db.show('plugin'):
            db.load('plugin', plugin)