Process overload-general descriptor line

As part of relay-search#40005 (closed) we want to expose an overload flag on relay-search so that relay operators know when their relay is experiencing an "overloaded state".

While metrics-lib is already exposing overload-ratelimits and overload-fd-exhausted flags, these are both in the extrainfo descriptor. Processing these lines in onionoo needs a little bit of planning to bump more memory to the onionoo machines.

As discussed in onionoo!2 (comment 2746623) this change can happen in 2 moments.

First we expose the overload-general line from the server descriptor [1] to add a boolean attribute that will alert operators that their relay is in an overloaded state

Then we plan a memory boost to the onionoo machines or a better update strategy for the details document and deploy the change discussed in onionoo!2 (closed)

[1] https://github.com/torproject/torspec/blob/main/dir-spec.txt#L638