[bridge 57/n] enum-dispatch BridgeActions and move encoding logic to …

…its own file (#16478)

## Description

For reviewers: not much to read in this PR, mostly moving things around.

## Test Plan

How did you test the new or updated feature?

---
If your changes are not user-facing and do not break anything, you can
skip the following section. Otherwise, please briefly describe what has
changed under the Release Notes section.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

crates/sui-bridge/Cargo.toml

@@ -47,6 +47,7 @@ shared-crypto.workspace = true
 backoff = { version = "0.4.0", features = [
   "futures",
 ] }
+enum_dispatch.workspace = true
 sui-json-rpc-api.workspace = true
 
 [dev-dependencies]