Post review changes to #74

[jamienoss]

Improvements to #74 Resolves #67

• Adds save and load methods to evolver.base.BaseConfig
• Separate settings for application settings.
• Reading the docs, it's now better to use a lifespan event rather than individual startup and shutdown events.
• Instantiate app evolver instance on startup, not at module level. (This required #67 to be resolved - see https://github.com/ssec-jhu/evolver-ng/issues/67#issuecomment-2135687714)

See https://github.com/ssec-jhu/evolver-ng/pull/74#issuecomment-2135456108

I think the files should rep descriptors not just the config such that the file also has the classinfo so that when looking at a config file you know what model it's for.

Also, we should note that pydantic has similar functionality for reading from files YamlConfigSettingsSource but isn't quite what we want.

[jamienoss]

@amitschang converted this back to a draft for a mo, whilst I sort out the tests and config file.