(nit) add symlink to make binary formats more discoverable (#3611)

## Motivation

Tracking binary formats is important but the corresponding file is
buried in the directory tree and looks like just another test snapshot.

## Proposal

* Create a symlink to make the file more visible

This more stable link could also be used later for `serde_generate`

## Test Plan

Nothing to do

Author: ma2bd

linera-rpc/binary_formats.yaml

@@ -0,0 +1 @@
+tests/snapshots/format__format.yaml.snap