client: make InternalAuth.Name omitempty to allow @anoninternal auth

In service of
https://lists.sr.ht/~sircmpwn/sr.ht-dev/%3C5webr4rzb2eulturkipecdkwms6m7kk2yzafse4p7jybaoknzz@tarta.nabijaczleweli.xyz%3E#%3C56tsvmov5pbxdinyz22rsiq4xzg2k47egmkgdj64fegphk2kvf@tarta.nabijaczleweli.xyz%3E

client: add User-Agent to outgoing requests

graphql: add support for multipart file uploads

Inspire from the python implementation in core.sr.ht.

Link: https://git.sr.ht/~sircmpwn/core.sr.ht/tree/7bcbc8ba8f4e/item/srht/graphql/client.py
Signed-off-by: Robin Jarry <robin@jarry.cc>

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.

client/graphql: handle GraphQL errors

Instead of leaving it up to the caller to check for GraphQL errors,
centralize in core-go.

go fmt

client: add config option to override API origin

If the config file contains an "api-origin" key for a service, prefer it
instead of "origin". This allows users to directly point the Python
frontend to the Go backend for local development, without having to
setup a reverse proxy.

This is the Go port of [1].

While at it, use GetOrigin for symmetry with the Python code.

[1]: https://git.sr.ht/~sircmpwn/meta.sr.ht/commit/6c2ee03923fe65423a2c55e6648f555c08db827a

client: populate NodeID from service name

Drop leftover references to gql.sr.ht

client: import from gql.sr.ht