feat: support xrpc custom healthcheck methods

[AlinsRan]

By adding callback functions, custom health check methods can be applied to private protocols.

local checker = healthcheck.new({
  name = "testing",
  shm_name = "test_shm",
  type = "xrpc",
  checks = {
      active = {
          healthy  = {
              interval = 0.5, -- we don't want active checks
              statuses = {200,201},
              successes = 2,
          },
          unhealthy  = {
              interval = 0.5, -- we don't want active checks
              statuses = {500, 501},
              failures = 2,
          },
          xrpc_handler = function(node, conf)
              local http = require('resty.http')
              local httpc = http.new()
              local res, err = httpc:request_uri("http://127.0.0.1:2119/status", {
                  method = "GET",
                  path = "/status",
              })
              if not res then
                  return false
              end
              return true, res.status
          end
      },
  },
})

[CLAassistant]

All committers have signed the CLA.

[membphis]

we'd better add xrpc, it is meaningful

[AlinsRan]

The resty.http is only used in testing.
After merging the PR here, the healthcheck library will be released and the xrpc suffix will be added.

[AlinsRan]