Give libbrpc a SONAME (ABI-stability)

**Is your feature request related to a problem?**

According to Debian Policy guidelines, it's not realistic to ship a libbrpc without a versionned SONAME in `/usr/lib/${ARCH}/`; see https://manpages.debian.org/sid/dpkg-dev/dpkg-gensymbols.1.en.html#Good_library_management

**Describe the solution you'd like**

It'd be ideal for libbrpc to have ABI stability guaranteed by a SONAME which would only get bumped at major versions.

**Describe alternatives you've considered**

Making libbrpc a private library imposes that all consumers of libbrpc need to be statically compiled, or in tightly-coupled versions; that's not really sustainable.

Start by reviewing brpc's library build and packaging configuration, since the issue names no specific file or test. Read the linked Debian Policy guidance and determine how the shared library is installed under /usr/lib/${ARCH}/. Done means libbrpc has a versioned SONAME with a defined major-version bump policy and packaging remains compatible.

Written by the indexing model from the issue text.