Added low-level mechanism to run weaver apps.

[mwhittaker]

This PR introduces a new low-level mechanism, run.Run, to run a weavelet when the topology of the application (e.g., the address of every listener and component) is static and predetermined.

To draw an analogy, git distinguishes between plumbing and porcelain. Plumbing commands are complicated low-level commands. Porcelain commands are simple, high-level commands that are built on top of plumbing commands. Most users use porcelain commands most (or all) of the time, but plumbing commands exist to make more complex git operations possible (though not necessarily convenient).

For Service Weaver, weaver single, weaver multi, weaver ssh, weaver kube, and weaver gke are porcelain commands. We don't offer any plumbing commands, besides writing a deployer completely from scratch. This PR aims to introduce a generally useful plumbing command.

Motivation

Here's our vision for how this PR will be useful. The details are not final, but I want to share something concrete to motivate the PR. We'll add a user-facing plumbing command on top of the run package, let's say weaver run. weaver run will take a topology file describing the topology of the application, say topo.toml:

deployment_id = "v1"
topology = {
    "Main": ["localhost:9000"],
    "Odd": ["localhost:9001"],
    "Even": ["localhost:9002"],
}
groups = [
    {
        name: "main",
        components: ["Main"],
        listeners: {"collatz": "localhost:8000"},
        address: "localhost:9000",
    },
    {
        name: "odd",
        components: ["Odd"],
        address: "localhost:9001",
    },
    {
        name: "even",
        components: ["Even"],
        address: "localhost:9002",
    },
]

Then, you can run an application manually like this:

$ weaver run weaver.toml topo.toml odd  # run "odd" group
$ weaver run weaver.toml topo.toml even # run "even" group
$ weaver run weaver.toml topo.toml main # run "main" group

This command will be generally useful, but our motivating use case is to enable people to run Service Weaver applications on Kubernetes with a level of control not possible with weaver kube.