kms-vault: Support HTTP redirect for Vault requests.

[chris-giblin]

As discussed in PR #51 (comment), HTTP redirect responses (3xx) are not currently supported but need to be. The goal of this issue is to implement redirects as required by Vault.

Background information.

• HTTP redirects are legitimately used when Vault is deployed in a High Availability configuration.
• Current Vault KMS provider makes explicit, precise check for HTTP status 200 on Vault responses (see PR discussion linked above).
• The HTTP client currently is not created with a redirect policy. As a result, redirects will not be followed. Because of the previous point, the redirect status code will be compared against 200 and be treated as an error.
• Consider supporting a KMS provider-specific configuration parameter such as follow-redirects with values true|false, corresponding to the HTTP client Always|Never settings in the HttpRedirect policy, so that redirection can be enabled or disabled.
• Using the HttpRequest.Builder, create the HTTP client under consideration of the configured redirect policy.