Refactor mysql server for runtime

[xuanyuan300]

What's Changed:
The goal of this refactor is to improve the scalability of the runtime, Users can implements their own handling logic.
The important changes as follows.

Trait:

1. Add MySQLService trait to handle the command for mysql protocol.
   Any third-part can be implements MySQLSerivce.

Struct:

1. Add MySQLInstance struct to start a MySQLService instance, it can be execute the method of MySQLSerice trait.
2. Add PisaMySQLService struct to implements the MySQLService trait as a default implemenets in the Pisa-Proxy internal.

Codec:

1. Add HandshakeCodec to parsing mysql handshake packet.
2. Add PacketCodec to parsing mysql protocol.

Note:
This pr also fix some unused result warn.

[codecov]

Codecov Report

Merging #274 (b80b706) into master (df61eb4) will increase coverage by 3.32%.
The diff coverage is 79.68%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
+ Coverage   66.97%   70.30%   +3.32%     
==========================================
  Files           7        6       -1     
  Lines         427      495      +68     
==========================================
+ Hits          286      348      +62     
- Misses        133      137       +4     
- Partials        8       10       +2     

Flag	Coverage Δ
unittests	70.30% <79.68%> (+3.32%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pisa-controller/pkg/webhook/server.go	26.66% <0.00%> (ø)
pisa-controller/pkg/proxy/http.go	58.62% <53.84%> (+0.72%)	⬆️
pisa-controller/pkg/webhook/injection.go	59.02% <82.56%> (+20.08%)	⬆️
pisa-controller/pkg/proxy/builder.go	91.21% <100.00%> (ø)

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