Allow way to not use manifest file with advanced config

Currently it seems that when using an advanced configuration with a `release-please-config.json` the only option is to use a `release-please-manifest.json` to manage the version number

I see the advantage of using a manifest for monorepos that manage multiple application releases. But it brings limitations for projects with a single version number

For example, it requires that whenever creating a new release tag the release please manifest must also be updated. This is problematic for workflows that use release please to manage prereleases, but have a separate workflow to bump production tags.

For example
Release please tags a release as v1.1.0-beta. The release please manifest reflect the latest tag
v1.1.0-beta is promoted and tagged as v1.1.0 using a separate flow. The release please manifest no longer reflect the latest tag
The next release please version will be v1.1.0-beta.1 rather than v1.2.0, as release please is unaware of the new v1.1.0 version

It would be useful if there was a way to use latest tags, rather than relying wholly on the release please manifest

Start by tracing how advanced configuration reads release-please-config.json and how the current version is obtained from release-please-manifest.json. Compare that flow with the latest release tags, including prerelease and promoted production tags. Done means a supported configuration can use tags without requiring the manifest while preserving existing manifest behavior.

Written by the indexing model from the issue text.