[confmap] Allow using any type as a string

[mx-psi]

Description

Allows any type to be used as a string if the target field is a string or the value is expanded in inline position. Inspired by #10794. This is not a breaking change (previously we would return an error for these).

Some things that you can do after this PR that you couldn't do before it:

1. Set HOST to [2001:db8::8a2e:370:7334] and use it in an endpoint:

exporters:
  otlp:
    endpoint: http://${env:HOST}/api/v1/traces

2. Pass really big numbers as strings (e.g. 10000000000000000000)

3. Pass null as a string.

Types that can be returned by our current YAML parser

Our current way of using the YAML parser has these types: string, nil, int, uint64, float64, map[any]any, map[string]any, []any.

There is no documentation for this, but the following fuzzing test did not find any failing cases after 20 minutes of continous run:

package main

import (
	"testing"
	"gopkg.in/yaml.v3"
)

func FuzzTest(f *testing.F) {
	f.Fuzz(func(t *testing.T, data []byte){
		var b any
		err := yaml.Unmarshal([]byte(data), &b)
		if err != nil {
			return
		}

		switch b.(type) {
			case string, nil, int, uint64, float64, map[any]any, map[string]any, []any:
				return
			default:
			  t.Errorf("Unexpected type %T", b)
		}
	})	
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.71%. Comparing base (7c2002a) to head (b2e8804).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10800      +/-   ##
==========================================
+ Coverage   91.70%   91.71%   +0.01%     
==========================================
  Files         406      406              
  Lines       18947    18947              
==========================================
+ Hits        17376    17378       +2     
+ Misses       1213     1209       -4     
- Partials      358      360       +2     

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