feat(proc, stub): expose GetMarkStubAddr API to scan and record memory partially by intrusively insert stub address in struct definition

[Lslightly]

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feat

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暴露GetMarkStubAddr API，通过在结构体定义中侵入式插入桩地址实现内存部分扫描和记录

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en: The most significant modification is in pkg/proc/reference.go. A markContext is introduced to indicate whether an object or a field can be recorded. markContext.useMarkStub is always true/false depending on whether --usemarkstub option is enabled. markContext.enableRecord is true when the stub address is found or an object is referenced directly/indirectly by other objects which contain the stub address.

The testcase is test/testdata/stubaddr/main.go. There are two sessions, one session with stub address while markStub of the other one is nil. Only the former session's objects will be recorded using the command grf attach <pid> --usemarkstub

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Fixes #71

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