Implementation for optional SEPA credit transfer purpose codes

[linushstge]

This pull requests implement the SEPA purpose codes to flag transactions for specific payment types, like salary payments.

Full list of SEPA codes:
https://wiki.windata.de/index.php?title=Purpose-SEPA-Codes

Additional optional parameter $purposeCode added to the credit transfer builder.
Fully backwards compatible

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Additional changes:

• Added PHP 8.3 to readme (tested in production)
• $endToEndId in CustomerInstantCreditTransferBuilder is null on default (same behaviour as CustomerCreditTransferBuilder)

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Generated SEPA XML Sample for SALA Code:

<CdtTrfTxInf>
    <PmtId>
        <EndToEndId>**********</EndToEndId>
    </PmtId>
    <Amt>
        <InstdAmt Ccy="EUR">100.00</InstdAmt>
    </Amt>
    <Cdtr>
        <Nm>Payment Receiver</Nm>
    </Cdtr>
    <CdtrAcct>
        <Id>
            <IBAN>DE****************</IBAN>
        </Id>
    </CdtrAcct>
    <Purp>
        <Cd>SALA</Cd>
    </Purp>
    <RmtInf>
        <Ustrd>Purpose Description</Ustrd>
    </RmtInf>
</CdtTrfTxInf>

[guillaume-sainthillier]

LGTM @andrew-svirin