Add `getFieldValue` and `setFieldValue` to `ClassMetadata` interface #873

Job	Run time
PHPUnit / PHPUnit (8.1, highest)	21s
PHPUnit / PHPUnit (8.3, highest)	24s
PHPUnit / PHPUnit (8.2, highest)	22s
PHPUnit / PHPUnit (8.5, highest)	28s
PHPUnit / PHPUnit (8.4, highest)	16s
PHPUnit / PHPUnit (8.1, lowest)	25s
PHPUnit / Upload coverage to Codecov	10s
	2m 26s

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