Backport config type fixes to 1.20.x

[sagikazarmark]

See #2003
See #2007

[Copilot]

Pull Request Overview

This PR backports config type fixes to the 1.20.x branch by changing how the configuration type is determined and how configurations are read and merged. Key changes include:

• Removing explicit checks for config type in ReadConfig and MergeConfig and deferring this to unmarshalReader.
• Replacing direct assignment of a new map with reading into a temporary config map.
• Introducing new tests that set a dummy config file to infer configuration type from its file extension.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
viper.go	Refactors ReadConfig, MergeConfig, and unmarshalReader to improve config type handling.
viper_test.go	Adds tests for ReadConfig and MergeConfig when a config file is set to derive config type.

Comments suppressed due to low confidence (1)

viper.go:1673

• Consider checking the error returned by buf.ReadFrom(in) to handle potential read failures.

buf.ReadFrom(in)