auth: add auth.IPAddress

With support for X-Forwarded-For

auth/middleware: set WWW-Authenticate header field

See RFC 6750 section 3.

client: rename Execute to Do

583d0b1bcb08 ("client/graphql: handle GraphQL errors") changed
Execute behavior by wrapping the result into a struct with "data"
and "errors" fields. This is a breaking change, but it's hard to
spot when upgrading core-go because it won't cause a compilation
error.

Rename Execute to Do to break the build and force callers to update
accordingly.

auth: add RequireMiddleware

Same as Middleware, but requires auth for all requests. Will be
useful to drop hacks from pages.sr.ht.

Drop gqlparser v1 dependency

Only use v2 throughout the repository.

auth: make DecodeGrants return an error

We'll use this function to validate grants passed in via
builds.sr.ht manifests.

auth: add AuthContext.Access

Same logic as server.Access, but lower-level. Useful to check for
a permission not covered by the GraphQL schema @access directives
(such as builds.sr.ht secrets).

auth: add /query/external/* to anonymous whitelist

auth/middleware: make auth scheme case-insensitive

According to RFC 7235 section 2.1, the auth scheme is
case-insensitive.

auth: fix invalid json tag in InternalAuth

auth: Use canonical user IDs

When adding users to the database, use the canonical user ID from
meta.sr.ht.

Revert "auth: Use canonical user IDs"

This reverts commit 6c1036e2705d93e1cb71476ac78499ae27f0d326.

auth: Use canonical user IDs

When adding users to the database, use the canonical user ID from
meta.sr.ht.

Fix (some) user PGP key lookups from meta.sr.ht

The authForUsername() function uses a closure that is almost identical
to the LookupUser() function, but is missing the handling for the PGP
key if called from meta.sr.ht. This causes at least the email
notifications for new OAuth2 tokens to be sent unencrypted. This commit
fixes that (and reduces code duplication) by calling LookupUser()
instead.

This requires the context for the tests to have a value for the calling
service, so add that to the mock.

go fmt

auth: fix nil reference in auth method check

Add @anoninternal support code

This is a slightly better approach to the previous commit.

auth: implement internal anonymous authentication

This is used for meta.sr.ht's GraphQL user registration resolver, which
needs to run prior to the user's information being added to the database.

go fmt

Fix json struct tag

Found via `go vet`:

    auth/middleware.go:259:2: struct field tag `json:"oauth_client_id",omit-empty` not compatible with reflect.StructTag.Get: key:"value" pairs not separated by spaces