PBS-Go: Use global objects through shims to be able to swap implementations

[justadreamer]

Note: This is PBS-Go-specific. Proof-of-concept PR: #4085

Motivation

Be able to swap dependencies to test and customize behaviors.

Rationale

The primary use case that can benefit from this is logging. prebid-server code makes direct calls to glog.Warningf, glog.Errorf. This is a side-effectful behavior that depends on glog package. It would be useful to encapsulate it behind some common logging interface facade to be able to replace glog with another logger in compile time to a) be able to test b) f.e. use other logger s.a. slog

The example of how it can benefit testing is given as part of solving this issue: #3748 with this PR: #3829. The issue specifically complains that there is undesirable logging happening and it would be nice to remove it. To actually test that it has been removed - we need to inject a different logger during testing - this is done using an interface and instance provided in di/logger.go and tests instantiating a different instance in runtime.

Another example is that if logger is encapsulated - we will be able to easily swap the glog with slog f.e. for the use case described here: #4062

Mechanism

Currently this is only done for logger object under logger package.
logger/interface.go defines the interface. The providers (implementers of the interface) are logger/default.go and logger/alternative.go - one of them can be compile-time enabled (using build tags). The provider is instantiated in logger/logger.go. Alternative implementation of logger can be built with a tag custom_logger likego build -tags custom_logger.

The call site will simply replace glog.X call with logger.Log.X.

Proof-of-concept

PR: #4085