Configurable namespace prefix for `ZSTD_*` APIs

**Is your feature request related to a problem? Please describe.**

Could we support configurable namespace prefix for `ZSTD_*` APIs (something like `--with-jemalloc-prefix=` in jemalloc (https://github.com/jemalloc/jemalloc/blob/dev/INSTALL.md)) to allow embed two (maybe different) zstd library in two separated components of a single application and avoid possible symbol conflict?

**Describe the solution you'd like**

Add a macro like `ZSTD_NAMESPACE` which defaults to `ZSTD_` but can be overrided and makes all APIs in zstd depends on it.

**Describe alternatives you've considered**
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**Additional context**
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