Add `vendored` feature to zstd-sys for providing pre-built zstd libs

[abrisco]

I would like to be able to provide my own libzstd.a when building zstd-safe. This change introduces the vendored feature in combination with the following environments to support this:

• ZSTD_SYS_VENDORED: An extra flag to enable the vendored search even if the feature is not enabled
• ZSTD_SYS_VENDORED_LIBS: A space delimited string of paths to compiled zstd libraries
• ZSTD_SYS_VENDORED_INCLUDE: The path to the zstd include directory where headers like zstd.h can be found.

[abrisco]

The naming of the feature was inspired by https://docs.rs/openssl/latest/openssl/#vendored

[gyscos]

Hi, and thanks for the report!

Is there any way this could be done with the current pkg_config feature?