Add Decoder::getTXTRecordFromDomain()

[BrandonXLF]

This PR moves SPF TXT record finding functionality to its function, getTXTRecordFromDomain.

For my project, I would like to have access to the raw DNS text record to show to users whenever possible. Currently, this is not possible since TXT record finding happens within getRecordFromDomain, so it's impossible to access the raw TXT record outside of that function.

With the PR, the following is possible:

$decoder = new \SPFLib\Decoder();
$rawRecord = null;

try {
	$rawRecord = $decoder->getTXTRecordFromDomain("example.com");
} catch ( \Exception $e ) {
	// Handle raw TXT record-related error.
}

$record = null;

if ($rawRecord) {
	try {
		$record = $decoder->getRecordFromTXT($rawRecord);
	} catch ( \Exception $e ) {
		// Handle recording parsing-related error.
		// Has access to $rawRecord to provide further context.
	}
}

// Has access to both $rawRecord and $record.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.76%. Comparing base (7312918) to head (4da1fd5).
Report is 1 commits behind head on main.

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Additional details and impacted files

@@             Coverage Diff              @@
##               main      #40      +/-   ##
============================================
+ Coverage     92.74%   92.76%   +0.02%     
- Complexity      662      664       +2     
============================================
  Files            44       44              
  Lines          1598     1603       +5     
============================================
+ Hits           1482     1487       +5     
  Misses          116      116              

Flag	Coverage Δ
unittests	92.76% <100.00%> (+0.02%)	⬆️

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[mlocati]

@BrandonXLF I've updated this PR with 4da1fd5

[mlocati]

Thanks! I've just published version 3.3.0 with this change.