hidden: add support to explicitly access hidden changesets via HTTP We now detect hidden access request and validate them while handling HTTP wireprotocol command. This changeset only introduces server support. Client support is added in a subsequent changeset. The mechanism we use for the HTTP wireprotocol is the same as the one used to lift filtering for hgweb. The use of the global `access-hidden` parameter in the http url is a simple and effective approach that * works around the lack of global parameters in wire protocol v1, * reuses the mechanism used for hgweb (that does not use the wireproto), and * can be safely ignored by older client (fitting the best effort contract). The feature is still experimental, so we can easily update the implementation details in the future. However, landing a first implementation now will help test the feature in the wild and iterate on alternative implementations if deemed necessary.

hidden: support for explicitly accessing hidden changesets in wireproto server This installs the basic infrastructure to be able to access hidden changeset through the wireprotocol. Each wireprotocol server still needs a way to define how the feature is triggered.