tests: add catchpoint downloading/parsing to e2e catchup tests

[cce]

Summary

While working on #6214 and #6177, it would have been nice to be able to download catchpoint files and decode them inside E2E tests. This exports the catchpoint chunk formats (and msgp unmarshal code), so they can be used by the e2e tests to decode and assert catchpoint file snapshot contents.

While renaming the chunk types so they would be exported, their new names were carefully chosen to preserve alphabetical ordering and make the diff in msgp_gen.go easy to review (to see it is a name change only, no other changes to decode/encode logic).

Test Plan

Existing E2E tests should pass, with additional assertions added.

[cce]

Running these assertions on TestStateProofInReplayCatchpoint but with the fix in #6214 commented out, it caught the issue:

    catchpointCatchup_test.go:539: Downloading catchpoint file for round 28 from http://127.0.0.1:59582/v1/test-v1/ledger/s
    catchpointCatchup_test.go:545: Downloaded catchpoint file for round 28, size 10240 bytes
    catchpointCatchup_test.go:552: tar filename: content.msgpack, size 312
    catchpointCatchup_test.go:552: tar filename: stateProofVerificationContext.msgpack, size 125
    catchpointCatchup_test.go:552: tar filename: balances.1.msgpack, size 511
    catchpointCatchup_test.go:560: chunk 1 has balances: 4, kvs: 0, online accounts: 0, onlineroundparams: 0
    catchpointCatchup_test.go:552: tar filename: balances.2.msgpack, size 3936
    catchpointCatchup_test.go:560: chunk 2 has balances: 0, kvs: 0, online accounts: 17, onlineroundparams: 0
    catchpointCatchup_test.go:552: tar filename: balances.3.msgpack, size 906
    catchpointCatchup_test.go:560: chunk 3 has balances: 0, kvs: 0, online accounts: 0, onlineroundparams: 15
    fixture.go:120:
                Error Trace:    /Users/cce/ga9/test/e2e-go/features/catchup/catchpointCatchup_test.go:529
                                                        /Users/cce/ga9/test/e2e-go/features/catchup/stateproofsCatchup_test.go:98
                Error:          Not equal:
                                expected: 8
                                actual  : 15
                Test:           TestStateProofInReplayCatchpoint
                Messages:       online round params chunk should be same size as lookback 8

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 51.85%. Comparing base (c60db8d) to head (e16e00e).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
ledger/catchupaccessor.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6224      +/-   ##
==========================================
- Coverage   51.86%   51.85%   -0.02%     
==========================================
  Files         643      643              
  Lines       86382    86382              
==========================================
- Hits        44806    44796      -10     
- Misses      38714    38724      +10     
  Partials     2862     2862              

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[algorandskiy]

I'd prefer not to export chunks internals out of ledger to prevent dependencies growing given there is already multi-version confusion.
How about making the validateCatchpointChunks an internal exported function on some some ValidatableCatchpointChunk interface?

[cce]

So move it out of _test.go code? Not sure what the best design is — I need to decode and validate the chunks directly in my test. Maybe I could have a decoder interface method turn it into a JSON style map or something if we don't want chunks exported.