PHP: reject metadata keys that are not legal

[stanley-cheung]

Fixes #7895

[stanley-cheung]

Now the C extension rejects any metadata key that fails the grpc_header_key_is_legal check. For the normal code path, the return false case has already been handled. For the plugin_get_metadata callback, an error code is now being returned to core via the grpc_credentials_plugin_metadata_cb callback.

Since it's now the caller's responsibility to make sure metadata keys are lower case, I keep the strtolower conversion in the per_rpc_creds interop tests.

PTAL again, thanks!

[murgatroid99]

I don't think it needs to be in this change, but now that you're validating keys, it would probably also be a good idea to validate metadata values.

Otherwise, LGTM