Create a middleware that allows executing OPA policies prior to invoking
the function.

The policy can be configurd via the `OPA_POLICY` environment variable.
The query is defined via the `OPA_QUERY` variable. The OPA_POLICY can be
a comma separated list of paths.

The default query input will contain `path`, `method`, and the
`authorization` header value. You can optional include the parsed `body`
`rawBody` and the raw `headers` via the `OPA_INCLUDE_JSON_BODY`,
`OPA_INCLUDE_RAW_BODY` and `OPA_INCLUDE_HEADERS` environment variables.

Signed-off-by: Lucas Roesler <[email protected]>

Add `OPA_SKIP_PATHS` configuration and update the implementation to skip
these paths.

chore: add HMAC auth example in the tests

Signed-off-by: Lucas Roesler <[email protected]>

Signed-off-by: Lucas Roesler <[email protected]>

Signed-off-by: Lucas Roesler <[email protected]>

Support parsing structured auth policy results. This allows the policy
to customize the error response code. The policy can also attach
additional headers to the request so that the implementation. The
primary use case for the additional headers is to pass user or client
identification to the implmentation to allow for auditing and/or
additional authorization checks.

Signed-off-by: Lucas Roesler <[email protected]>