allow users to specify a prebuilt reqwest client

[tofay]

if specified this will be used instead of dkregistry creating one.

This makes it easier to use non-standard authentication mechanisms:

• authenticating with Artifactory registries by setting a custom header
• setting the Authentication header when interacting with Azure Container Registries.

(In the latter case users login using az tool which writes an oauth2 token to the identitytoken field of the relevant auth section of $HOME/.docker/config.json. When communicating with the registry that token can just be provided by adding the header Authentication: Bearer <token> )

[tofay]

In both my use cases I want to build a reqwest client that adds a specific header, so maybe I should be allowing users to specify additional headers instead, to avoid putting reqwest::Client into dkregistry's public API? (I think the use of reqwest is an implementation detail right now)

[vrutkovs]

maybe I should be allowing users to specify additional headers instead

Yeah, I think that would be more flexible and avoid possible API issues. IIUC #212 is implementing that