openapi3filter: don't consume request body in AuthenticatorFunc

[jamietanna]

As noted in #743, some users of the openapi3filter, largely via
oapi-codegen and openapi.tanna.dev/go/validator0 are seeing that
when using an AuthenticationFunc, this can lead to errors such as:

request body has an error: value is required but missing

Or:

request body has an error: reading failed: http: invalid Read on closed Body

This turns out to be down to the fact that a call to the
AuthenticationFunc may read the request body, but due to the way that
Go's io.Readers can only be read once, this then means the body cannot
be read by anyone else further in the request chain.

To resolve this, we can make sure that each iteration of the
AuthenticationFunc gets a fresh copy of the request body (in its own
io.Reader).

This follows logic that exists within the ValidateRequestBody method,
with an additional layer of caching the raw bytes from the body.

We also add tests to validate:

• calling the ValidateRequest body function with an
  AuthenticationFunc that reads the request body
• using an integration test with a test HTTP server also works,
  indicating that the HTTP handler can then read the request body itself

Closes #743.

[fenollp]

Thanks!