Extend configuration to set maximum claim size

[letmaik]

Currently, an upper limit of 1 MiB is hard-coded. This should be configurable, defaulting to 1 MiB.

Generally, the idea is still that the ledger should not contain large content, so setting this to 1 GiB or similar is discouraged. The solution to this is to reference content stored elsewhere (by hash) instead of embedding it.

[ivarprudnikov]

• Reference to our hardcoded value https://github.com/microsoft/scitt-ccf-ledger/blob/main/app/src/constants.h#L14
• Reference to the default limit in CCF https://github.com/microsoft/CCF/blob/main/include/ccf/http_configuration.h#L14
• Reference to CCF doc https://microsoft.github.io/CCF/main/operations/generated_config.html#http-configuration

[achamayou]

In addition to the size limit, it may be useful to let a ledger instance define that it only accepts COSE Hash Envelope by checking for the presence of 258 in protected headers: https://datatracker.ietf.org/doc/draft-ietf-cose-hash-envelope/

It is not a perfect substitute to the overall size cap per request, which is still necessary in case of large COSE headers, and in case the payload is not a genuine COSE Hash Envelope payload. It might still result in better UX though, because users can be made aware that they should submit hash envelopes upfront, rather than submit payloads that may grow over time and painfully discover the limit at an inconvenient time.