Allow generator registrations to specify a starting point

[johnsonr]

Builds on #424 - please review after that

Allow GeneratorRegistration to include a startingPoint, which can be a project or a seed repo. This is an alternative to using the source on the parameters object.

This clarifies how generators work - starting point + transformation - and allows the starting point to be created in memory if desired.

In future, generators will probably be expressed directly this way rather than in function wrappers as in the Spring pack. (They can use the functions provided in such packs for their transformation.)

Examples:

sdm.addGeneratorCommand<SeedDrivenGeneratorParameters & { path: string }>({
        name: "foo",
        intent: "create thing",
        startingPoint: InMemoryProject.of(
            new InMemoryFile("README.md", "some content")),
        parameters: {
            path: {},
        },
        transform: async (p, ctx, params) => {
            return p.addFile(params.path, "1");
        },
    });

and:

sdm.addGeneratorCommand<SeedDrivenGeneratorParameters & { path: string }>({
        name: "foo",
        intent: "create thing2",
        startingPoint: GitHubRepoRef.from({owner: "spring-team", repo: "spring-rest-seed"}),
        parameters: {
            path: {},
        },
        transform: async (p, ctx, params) => {
            return p.addFile(params.path, "1");
        },
    });

Backward compatible.