topdown: support inline TLS certificates and keys

`http.send` already supports TLS keys and certificates obtained from
external files and environment variables. Add support for specifying
certificates and keys as direct parameters to `http.send`. This is
useful when you already have policy variables with this data.

There are 3 options that this affects: client certificates, client
keys and CA bundles. All of these can now be specified from either a
file, an environment variable or from raw data. In the case of client
certificates and keys, each access method must use a matched pair (e.g.
both the certificate and the key must come from environment variables).
CA certificate bundles are the union of all the specified access methods.

Signed-off-by: James Peach <jpeach@vmware.com>