feat: Add payer reports

[neekolas]

tl;dr

• Adds protos for serializable Payer Reports as a message type

[macroscopeapp]

Add PayerReport and PayerReportAttestation message types to XMTP v4 envelopes for payer reporting

Introduces new protocol buffer definitions in the xmtp.xmtpv4.envelopes package:

• Extends envelopes.proto with two new payload fields: PayerReport (field 6) and PayerReportAttestation (field 7)
• Adds payer_report.proto defining PayerReport message for tracking payer/node activity with merkle roots and PayerReportAttestation message for cryptographic verification

📍Where to Start

Begin review with the new message type definitions in payer_report.proto, followed by the payload field additions in envelopes.proto.

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Macroscope summarized 2595c22. (Automatic summaries will resume when PR exits draft mode or review begins).