Add function to create VaultClient with reqwest Client

[collinoc]

Recently ran into a scenario where I wanted to set the pool idle timeout on the inner client. I figure letting users supply their own reqwest client is easier than adding another field to VaultClientSettings anytime another nice-to-have comes up.

[stormshield-gt]

I think the direction we want to go now is to remove the public dependency on reqwest. The idea will be to provide better stability, i.e., avoid breaking when reqwest breaks. I will be open to a PR that adds the config options.

We can further discuss the reqwest situation here #152 if you want