unified logging pattern & facilities

Start by inspecting the salt/log package and its consumers to map the current Logrus/Zap abstraction, then check the mux package for logging-related middleware. Done means the formatted-logger abstraction is removed, Salt and ODPF usage assumes Zap structured logging, and any retained context helpers are explicitly justified.

Currently salt/log package tries to abstract Logrus and Zap into a common formatted-logger interface (i.e., Infof(msg string, args ...any), etc. ). While logrus is designed as a formatted-logger, uber/zap is specifically designed for efficient structured logging and this kind of abstraction nullifies the major benefit of it.

I propose we remove this abstraction altogether[^1] and assume direct usage of zap within salt and in ODPF applications that use salt. Benefits of doing this:

• All the benefits of structured logging (easy to parse logs, easy to search/filter by field values, easy to attach request context with each log, etc.)
• Not giving an abstracted formatted-logger will force us to always stick to structured logging.
• Assuming zap as the logger of choice allows us to provide certain useful utility abstractions. Few examples:
  * A request-logging middleware in mux package that automatically logs request info (method, path, client-ip, etc.) and response info (status, response time, etc.)
  * A middleware for injecting request related context (req-id, current user id, the route info, etc.) into req.Context() so that every log in all the subsequent layers automatically add this to every log entry.

[^1]: We can still have some utility functions if we need to (e.g., a helper to inject log context into ctx). But attempting to abstract over logging functionality will not have justifiable benefits.