[BATCHING] Implement batching with unpack/repack logic

[fredteumer]

Objective

Implement batching with unpack/repack logic. Unpack/repack is required so that suppliers can be correctly compensated with onchain mechanisms.

Origin Document

In the previous portal we supported this.

Goals

• Accept batch requests in the Portal via PATH
• Correctly compensate the Suppliers for each individual relay in a batch

Deliverables

• PATH correctly handles batch requests
• PATH unpacks the batch, processes each individual request and repacks the batch response for a user
• PATH correctly counts each individual request and submits it to the billing pipeline.

Non-goals / Non-deliverables

• Batch-based CUTTM
• Extra services that handle batch requests

General deliverables

• Comments: Add/update TODOs and comments alongside the source code so it is easier to follow.
• Testing: Add new tests (unit and/or E2E) to the test suite.
• Makefile: Add new targets to the Makefile to make the new functionality easier to use.
• Documentation: Update architectural or development READMEs; use mermaid diagrams where appropriate.

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Creator: @fredteumer
Co-Owners: @ArtSabintsev

[Olshansk]

From @fredteumer:

1. receive batch request
2. decompose into individual relays and process (each 1 is metered as a request in our system)
3. get responses and process (each 1 = one response in our system)
4. compose responses back into the batch response
5. send the user the response payload

@commoddity Please pick this up asap.

Some notes:

1. If you don't already use claude code, this is a good opportunity to help you kickstar the PR (this issue has everything you need).
2. This is a great use-case for new E2E tests
3. Please update our docs to reflect that we have support for it
4. Please add a make target to test it