Add a CommandCenterProvider to resolve CommandCenter API and cache the instance

[cdfive]

Describe what this PR does / why we need it

Cache the CommandCenter instance so that we can invoke any methods with it when needed.

Does this pull request fix one issue?

Fixes #345

Describe how you did it

Add a Class CommandCenterProvider which hold a CommandCenter instance, resolve the instance
by SPI and add a getCommandCenter() method to get the instance.

Describe how to verify it

Run sentinel-dashboard and make a breakpoint to test if CommandCenter loaded as expected.

Special notes for reviews

I found a useful method SpiLoader.loadFirstInstance(CommandCenter.class); to get the API，
which only get the first API instance,
but previous CommandCenterInitFunc checked "Only single command center can be started",
I think it's okay not to check the unique, since load first API can worked and user can also
check in sentinel-record.log.

[codecov-io]

Codecov Report

Merging #409 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #409   +/-   ##
=========================================
  Coverage     37.73%   37.73%           
  Complexity      982      982           
=========================================
  Files           235      235           
  Lines          7407     7407           
  Branches       1014     1014           
=========================================
  Hits           2795     2795           
  Misses         4253     4253           
  Partials        359      359

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[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!