Design a generic adaptor for Python vulnerability services

[woodruffw]

Now that we've evaluated osv.dev (#2), we can start designing an abstract interface that individual vulnerability services can satisfy.

From discussion on Slack: we probably want something like this:

class VulnerabilityService(ABC):
    def query(spec: Dependency) -> List[VulnerabilityResult]:
        ...

    def query_all(specs: List[Dependency]) -> Dict[Dependency, List[VulnerabilityResult]]:
        # naive implementation that can be overridden if
        # a particular service supports bulk queries
        for spec in specs:
            ...