Specify Vector version in the Vector config file

After talking with @a-rodin he had the following idea:

My thoughts about release process were mostly questioning the need to add version to the config file, similarly to how docker-compose files have version in the beginning. I see both benefits and downsides of this, but haven’t decided yet would it be better to have it or not.

He also mentioned:

I agree that it is possible to make breaking changes between 0.y.z releases according to semver. However, it could be nice to use configs from older versions in newer ones without manual changes, which can be done by running a special command which would take config from an older version and generate a config for the current version.

Something like running vector config upgrade which would bump the version in config and make necessary fixes, for example renaming config options or transforms names.

I don't think we should address this during the pre-1.0 phase of development, but I wanted to open this issue for discussion.

The issue names no files, tests, or entry points; start by resolving the open discussion about whether versioning belongs in configs before locating the configuration and CLI upgrade surfaces. Done would require an agreed versioning and upgrade scope, which the current issue does not define.