Add cets_status module

[arcusfelis]

Add cets_status module

The code gets a list of nodes, gets a list of servers and compares them.
After that it produces a map like:

-type info() :: #{
    %% Nodes that are connected to us and have the CETS disco process started.
    available_nodes := [node()],
    %% Nodes that do not respond to our pings.
    unavailable_nodes := [node()],
    %% Nodes with stopped disco
    remote_nodes_without_disco := [node()],
    %% Nodes that has our local tables running (but could also have some unknown tables).
    %% All joined nodes replicate data between each other.
    joined_nodes := [node()],
    %% Nodes that have more tables registered than the local node.
    remote_nodes_with_unknown_tables := [node()],
    remote_unknown_tables := [table_name()],
    %% Nodes that are available, but do not host one of our local tables.
    remote_nodes_with_missing_tables => [node()],
    remote_missing_tables := [table_name()],
    %% Nodes that replicate at least one of our local tables to a different list of nodes
    %% (could temporary happen during a netsplit)
    conflict_nodes := [node()],
    conflict_tables := [table_name()],
    %% Nodes that are extracted from the discovery backend.
    discovered_nodes := [node()],
    %% True, if discovery backend returned the list of nodes the last time we've tried
    %% to call it.
    discovery_works := boolean()
}.

The map could be used for getting the status of the cluster.

This functionality is used in MongooseIM like that esl/MongooseIM#4116

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.77% 🎉

Comparison is base (ada5e7e) 96.61% compared to head (7281810) 97.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   96.61%   97.38%   +0.77%     
==========================================
  Files           8        9       +1     
  Lines         531      612      +81     
==========================================
+ Hits          513      596      +83     
+ Misses         18       16       -2     

Files Changed	Coverage Δ
src/cets.erl	100.00% <100.00%> (ø)
src/cets_call.erl	87.09% <100.00%> (+2.48%)	⬆️
src/cets_discovery.erl	93.57% <100.00%> (+1.71%)	⬆️
src/cets_status.erl	100.00% <100.00%> (ø)

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[NelsonVides]

The set operations happening here over maps and lists and proplists and maps of proplists and lists of maps and maps of maps of lists and just about every combination possible, is actually quite entertaining :D

Just a few minor comments.

[NelsonVides]

LGTM