feat: use protobuf to store header and messages in ISB

[yhl25]

fixes #1768

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.40426% with 25 lines in your changes missing coverage. Please review.

Project coverage is 61.27%. Comparing base (4e88f32) to head (f05932e).

Files	Patch %	Lines
pkg/isb/serde.go	78.94%	14 Missing and 2 partials :warning:
pkg/isb/stores/simplebuffer/buffer.go	58.33%	4 Missing and 1 partial :warning:
pkg/isb/stores/redis/write.go	40.00%	2 Missing and 1 partial :warning:
pkg/isb/errors.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1771 +/- ## ========================================== + Coverage 60.83% 61.27% +0.43% ========================================== Files 232 231 -1 Lines 20014 19830 -184 ========================================== - Hits 12175 12150 -25 + Misses 6993 6886 -107 + Partials 846 794 -52 ```

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[vigith]

watermark propagation is still using encoding/binary; let's take that up too, next?

[yhl25]

any performance regressions?

i see that the WAL size can come down, so that is good :)

I didn't notice much difference with a low TPS pipeline. I have yet to perform some performance testing with a high TPS pipeline.