File-based Snouty configuration

[jeskew]

Closes #120

This PR adds support for reading configuration from two locations on disk in addition to the currently supported environment variables. All access to configuration is routed through a SnoutyConfig value that will try to source the tenant and container repository from the following locations on disk if the ANTITHESIS_TENANT or ANTITHESIS_REPOSITORY environment variables are not set:

• <project configuration directory>/snoutyconfig.toml
• $XDG_CONFIG_HOME/snouty/settings.toml/$HOME/Library/Application Support/com.Antithesis.Snouty/settings.toml

The first item is only supported for commands where the user is supplying a directory with a docker-compose.yaml file or kubernetes manifests, and the second item is in the global snouty config directory (as identified by the directories-next crate).

Both files have the same format, which looks like:

tenant = "your-tenant"
repository = "us-central1-docker.pkg.dev/your-project/your-repo"

profiles.bar.tenant = "another-tenant"

[profiles.foo]
repository = "us-central1-docker.pkg.dev/your-project/foo-repo"

Snouty will use the top-level tenant/repository values unless a profile is supplied via the ANTITHESIS_PROFILE environment variable. When we add OAuth support to Snouty, any requisite configuration will come from these files. API keys and basic auth are not supported via the config files, as those values are sensitive and thus shouldn't appear in a plaintext config file.

One thing I want to add that is not included in this PR is a snouty init command that will prompt the user for values and then write out config files for them. This PR was just feeling a bit large without it added, so I thought it made sense to split it out.

[jeskew]

@claude review

[carlsverre]

main review:

1. switch to .snouty.toml for project local config
2. Let's remove directories-next in place of just manually resolving XDG config dir
3. rebase