Add docs on how to implement a provider

[czechboy0]

Basic docs on https://swiftpackageindex.com/apple/swift-configuration/main/documentation/configuration/configprovider, but we should have a guide similar to https://swiftpackageindex.com/apple/swift-configuration/main/documentation/configuration/configuring-applications

[finnvoor]

FWIW I've implemented a TOML provider based on the existing YAML provider and it was quite straightforward, but there's a few package scoped types that I ended up copy and pasting into my package. It would be super useful if they were exposed in Configuration, or at least a separate target. I ended up needing to copy: CommonProviderFileSystem, LocalCommonProviderFileSystem, ConfigError, withConfigValueLookup(_:_:), InMemoryFileSystem, ReloadingFileProviderCore, and ReloadingFileProviderMetrics to support all the same features as the YAML one.

[czechboy0]

Hi @finnvoor - happy to hear you were able to implement a TOML provider. I agree that specifically ReloadingFileProviderCore would probably be very helpful to open up for other formats.

Did you end up having to change it in any way or did it work as-is?

[finnvoor]

Didn't have to change anything at all, just copy/pasted those classes/structs into a file in my package (see https://github.com/finnvoor/swift-configuration-toml/blob/main/Sources/ConfigurationTOML/utils.swift).

[czechboy0]

Hi @finnvoor, I'm starting to look into surfacing more of these building blocks as API, to hopefully reduce how much you had to duplicate in your package. I'll ping you when I have something ready to try, on a branch.