Include valkey_exporter into PMM Client distribution

We need to include redis_exporter into PMM Client distributions:

• DEB

• RPM

• Tarball

• Docker container

Binary name should be valkey_exporter.

For that we will need to do following:

• Create a spec file where code is loaded dynamically based on a tag or commit hash

• Create fork of redis_exporter (decided not to)

• Add new submodule to pmm-submodules that links to newly created fork (decided not to)

• Build redis_exporter under valkey_exporter name and create symlink

• Include the binary file to all distributions

QA

• install pmm client in all versions and check Valkey exporter binary

• check upgrade

• add to Autotests