MallocExtension declaration in tcmalloc_extension.h blocks UBSAN build

**Describe the bug (描述bug)**
[`MallocExtension`](https://github.com/apache/incubator-brpc/blob/dfe659ba364ab2ff59f94c70418d429904f2194f/src/brpc/details/tcmalloc_extension.h#L72) declaration in `brpc/details/tcmalloc_extension.h` is a (potentially mismatching) copy of `malloc_extension.h` provided by `google-perftools` package.

I understand that BRPC **intentially** left it there with no virtual methods defined, hoping provided by linking `libtcmalloc.so` provided by `google-perftools` package. Otherwise (_i.e._ when linking `jemalloc` or using default implementation), `MallocExtension` has no key function.

We can call it _WAI_ in most scenarios, but when not linking `tcmalloc` and in UBSAN build, a linking failure regarding `undefined reference to 'typeinfo for MallocExtension'` persists.

Moreover, the modern `tcmalloc` (greatly different from the one provided by `google-perftools` package) has a totally different definition of `MallocExtension`, so evening linking _this_ `tcmalloc` won't help.

I know that we can workaround this issue by `fno-sanitize=vptr`, but since `MallocExtension` is the **only** problematic type blocking linking, I would consider it's not worth the regression.

**To Reproduce (复现方法)**
Build a binary depending on `brpc` in UBSAN mode.

**Expected behavior (期望行为)**
Successful UBSAN build.

**Versions (各种版本)**
OS: Ubuntu 22.04
Compiler: Clang 12.0.1
brpc: 1.2.0
protobuf: 21.1

**Additional context/screenshots (更多上下文/截图)**
Pending.

Start with src/brpc/details/tcmalloc_extension.h, especially the MallocExtension declaration, and reproduce the link failure by building a brpc-dependent binary in UBSAN mode with the stated Ubuntu, Clang, and brpc versions. Compare the declaration with the google-perftools and modern tcmalloc definitions mentioned in the report. Done means the UBSAN build links successfully without requiring the fno-sanitize=vptr workaround.

Written by the indexing model from the issue text.