Extension (dubbo-common 2.6.4 API)

已过时。
because it's too general, switch to use SPI
```
@Deprecated
@Documented
@Retention(value=RUNTIME)
@Target(value=TYPE)
public @interface Extension
```
Marker for extension interface
Changes on extension configuration file
Use Protocol as an example, its configuration file 'META-INF/dubbo/com.xxx.Protocol' is changes from:
```
     com.foo.XxxProtocol
     com.foo.YyyProtocol
 
```
to key-value pair
```
     xxx=com.foo.XxxProtocol
     yyy=com.foo.YyyProtocol
 
```
The reason for this change is:
If there's third party library referenced by static field or by method in extension implementation, its class will fail to initialize if the third party library doesn't exist. In this case, dubbo cannot figure out extension's id therefore cannot be able to map the exception information with the extension, if the previous format is used.
For example:
Fails to load Extension("mina"). When user configure to use mina, dubbo will complain the extension cannot be loaded, instead of reporting which extract extension implementation fails and the extract reason.

可选元素
限定符和类型	可选元素和说明
`String`	`value` 已过时。

@Deprecated
public abstract String value

已过时。

注释类型 Extension