Custom serializer register

[Lo1nt]

Motivation:

Currently, the custom serializer is registered by static code block. And once it registered, it could not be modified.

When there are several classes that register custom serializer, the sequence, dependents on sequence of class loading, is unpredictable, and unmodifiable.

Thus, there's no way we define our own serializer (say, for SofaRequest.Class).

The pr here is aiming at making custom serializer registration sequentially.

P.S. And we here still don't want to change the unmodifiable feature (Once a serializer registered, it can never be changed).

Modification:

Apply extension-loader mechanism to let the register work in sequence.

Still, there is an override feature for extension loader, which enables us to substitute the origin register provided by sofa rpc.

Result:

1. Let all register become instance managed by ExtensionLoader.
2. Change call to static register method to get Extension instance and then call instances' register method.

[codecov[bot]]

Codecov Report

Merging #1296 (66cae3f) into master (a9af9cd) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1296      +/-   ##
============================================
- Coverage     71.95%   71.95%   -0.01%     
+ Complexity      784      783       -1     
============================================
  Files           415      416       +1     
  Lines         17654    17659       +5     
  Branches       2752     2752              
============================================
+ Hits          12703    12706       +3     
  Misses         3549     3549              
- Partials       1402     1404       +2     

Impacted Files	Coverage Δ
...in/java/com/alipay/sofa/rpc/common/RpcOptions.java	100.00% <ø> (ø)
.../rpc/codec/bolt/AbstractSerializationRegister.java	100.00% <100.00%> (ø)
...a/rpc/codec/bolt/SofaRpcSerializationRegister.java	81.81% <100.00%> (+9.09%)	:arrow_up:
...ipay/sofa/rpc/server/bolt/BoltServerProcessor.java	65.33% <100.00%> (+0.46%)	:arrow_up:
...y/sofa/rpc/transport/bolt/BoltClientTransport.java	85.88% <100.00%> (+0.17%)	:arrow_up:

... and 4 files with indirect coverage changes

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