chore: apisix http client

[tokers]

Add package pkg/apisix.

Add Interface apisix.APISIX as the unique APISIX client interface.

The simple use example:

cli, err := apisix.NewForOptions(&apisix.ClusterOptions{
    Name: "default",
    BaseURL: "http://127.0.0.1:9080/apisix/admin",
    AdminKey: "0x123fff"
})
if err != nil {
    log.Errorf("failed to create apisix client: %s", err)
    return
}

// Resource operations.
cli.Cluster("default").Route().List()
cli.Cluster("default").Route().Create(ctx, route)
cli.Cluster("default").Route().Update(ctx, route)
cli.Cluster("default").Route().Delete(ctx, route)

// Cluster oerations
cli.AddCluster(clusterOptions)
cli.ListClusters()

[codecov-io]

Codecov Report

Merging #147 (3f45041) into master (2a8e8a7) will increase coverage by 14.81%.
The diff coverage is 55.96%.

@@             Coverage Diff             @@
##           master     #147       +/-   ##
===========================================
+ Coverage   34.55%   49.36%   +14.81%     
===========================================
  Files          28       29        +1     
  Lines        1314     1333       +19     
===========================================
+ Hits          454      658      +204     
+ Misses        830      594      -236     
- Partials       30       81       +51     

Impacted Files	Coverage Δ
pkg/apisix/client.go	0.00% <0.00%> (ø)
pkg/seven/conf/conf.go	0.00% <0.00%> (ø)
pkg/seven/state/builder.go	0.00% <0.00%> (ø)
pkg/seven/state/service_worker.go	0.00% <0.00%> (ø)
pkg/seven/state/solver.go	3.38% <0.00%> (-0.12%)	⬇️
pkg/apisix/stub.go	37.31% <37.31%> (ø)
pkg/apisix/route.go	61.01% <61.01%> (ø)
pkg/apisix/ssl.go	63.79% <63.79%> (ø)
pkg/apisix/service.go	65.45% <65.45%> (ø)
cmd/ingress/ingress.go	78.35% <71.42%> (-0.54%)	⬇️
... and 11 more

---

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