feat(gql_dio_link): support per-request timeouts via DioLinkTimeoutCo…

[ljanecek]

Why

DioLink reads only DioLinkCancelTokenContextEntry and HttpLinkHeaders
from the request context — there is no way to set Dio's
sendTimeout/receiveTimeout for a single operation. Long-running
operations (e.g. a mutation waiting ~20 s for a device to acknowledge a
command) then require a second Dio/GraphQLClient instance, or smuggling a
marker header through HttpLinkHeaders and converting it in an interceptor.

What

Adds DioLinkTimeoutContextEntry({sendTimeout, receiveTimeout}), following
the same pattern as DioLinkCancelTokenContextEntry. null fields keep the
client-wide timeouts. Deliberately limited to timeouts (instead of a whole
dio.Options) so the link keeps control of responseType and headers —
happy to generalize if you'd prefer.

client.mutate(MutationOptions(
  document: ...,
  context: const Context().withEntry(
    const DioLinkTimeoutContextEntry(receiveTimeout: Duration(seconds: 20)),
  ),
));