api/client: provide the ability to set a logger on retryablehttp.Client

[calvn]

This PR adds the ability to set a logger on api.Client which is used on its underlying retryablehttp.Client instance when making requests. This value is nil by default and is only non-empty if set explicitly to maintain backwards compatibility and to avoid any unexpected output or noise that might result from it otherwise. Note that even though retryablehttp.Client accepts an interface{} as a logger to allow for either non-leveled or leveled loggers to be provided, I'm enforcing a leveled logger (e.g. hclog) to be provided to our api/client package.

As a concrete example, setting an hclog.Logger, which satisfies the retryablehttp.LevelLogger interface, on agent's proxy layer allows us to output the number of retry attempts that the client makes.

Without a logger:

2021-05-21T17:51:36.867-0700 [INFO]  auth.handler: authenticating
2021-05-21T17:51:37.209-0700 [INFO]  cache: received request: method=PUT path=/v1/auth/approle/login
2021-05-21T17:51:37.209-0700 [DEBUG] cache.leasecache: forwarding request: method=PUT path=/v1/auth/approle/login
2021-05-21T17:51:37.209-0700 [INFO]  cache.apiproxy: forwarding request: method=PUT path=/v1/auth/approle/login
2021-05-21T17:51:40.106-0700 [ERROR] auth.handler: error authenticating: error="Put "http://localhost:8200/v1/auth/approle/login": dial tcp [::1]:8200: connect: connection refused" backoff=1.98s
==> Vault agent shutdown triggered

With a logger:

2021-05-21T17:42:34.456-0700 [INFO]  auth.handler: authenticating
2021-05-21T17:42:35.032-0700 [INFO]  cache: received request: method=PUT path=/v1/auth/approle/login
2021-05-21T17:42:35.032-0700 [DEBUG] cache.leasecache: forwarding request: method=PUT path=/v1/auth/approle/login
2021-05-21T17:42:35.032-0700 [INFO]  cache.apiproxy: forwarding request: method=PUT path=/v1/auth/approle/login
2021-05-21T17:42:35.032-0700 [DEBUG] cache.apiproxy.client: performing request: method=PUT url=http://localhost:8200/v1/auth/approle/login
2021-05-21T17:42:35.033-0700 [ERROR] cache.apiproxy.client: request failed: error="Put "http://localhost:8200/v1/auth/approle/login": dial tcp [::1]:8200: connect: connection refused" method=PUT url=http://localhost:8200/v1/auth/approle/login
2021-05-21T17:42:35.033-0700 [DEBUG] cache.apiproxy.client: retrying request: request="PUT http://localhost:8200/v1/auth/approle/login" timeout=1.481137504s remaining=2
2021-05-21T17:42:36.517-0700 [ERROR] cache.apiproxy.client: request failed: error="Put "http://localhost:8200/v1/auth/approle/login": dial tcp [::1]:8200: connect: connection refused" method=PUT url=http://localhost:8200/v1/auth/approle/login
2021-05-21T17:42:36.517-0700 [DEBUG] cache.apiproxy.client: retrying request: request="PUT http://localhost:8200/v1/auth/approle/login" timeout=2.803235972s remaining=1
2021-05-21T17:42:37.757-0700 [ERROR] auth.handler: error authenticating: error="Put "http://localhost:8200/v1/auth/approle/login": dial tcp [::1]:8200: connect: connection refused" backoff=1.99s
2021-05-21T17:42:39.324-0700 [ERROR] cache.apiproxy.client: request failed: error="Put "http://localhost:8200/v1/auth/approle/login": dial tcp [::1]:8200: connect: connection refused" method=PUT url=http://localhost:8200/v1/auth/approle/login
==> Vault agent shutdown triggered