implement oauth2 HTTP client

[cataggar]

Fix #996. Twoazure_identity::token_credentials now require an Arc<dyn HttpClient>

• ClientSecretCredential
• EnvironmentCredential

The HttpClient is used to implement the oauth2 http client.

Tested with:

• cargo run -p azure_security_keyvault --example get_secret_versions
• cargo run -p azure_security_keyvault --example list_secrets

[rylev]

Looks good, just have some code quality comments.

@rylev, all your requested changes were made.

[rylev]

I was a little hesitant to merge this since I think things could change considerably if/when we move to using a pipeline, but this is an improvement over the status quo, so I think we should land it.