LPv2: Batch Message serialization

[lemunozm]

Description

This PR is related to the batch message support. The batch logic itself will be in a next PR.

Serialization Notes

We need a custom serialization/deserialization due to the spetial requirements of how batches needs to be serialized:

• REQ 1: Have the size of each submessage before the submessage as u16.
  • Technically, this is not needed for us because we know the structure of the message to deserialize.
  • Our workaround to get the message size is to serialize with our data format inside of the the serialization process.
• REQ 2: Not having a number indicating the size of the batch
  • Technically, this is required for us. We need to know the size of the batch and how many submessages we need to deserialize. Without that information means that we enter the deserialization logic with no idea of how to finish.
  • The workaround solution is to finish when we find an error (an "unexpected EOF") deserializing the next length of a submessage.

Changes

• [x] Custom Batch serialization/deserialization
• [x] Type system protection to avoid nested batches

Next PR

• Add Gateway::start_pack_batch() and Gateway::end_pack_batch()
• Gateway required changes (new storage to keep at least the boolean state of above)
• LPEncoding utilities to allow the Gateway to compose messages into batches without knowing the concrete Message type.

[lemunozm]

As a first step for the bench message feature, this should be ready.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.06452% with 26 lines in your changes missing coverage. Please review.

Project coverage is 46.15%. Comparing base (976eebe) to head (a771a96).

Files	Patch %	Lines
pallets/liquidity-pools/src/message.rs	54.71%	24 Missing :warning:
pallets/liquidity-pools/src/gmpf/error.rs	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feat/lp-v2 #1920 +/- ## ============================================== + Coverage 45.97% 46.15% +0.17% ============================================== Files 179 179 Lines 12791 12849 +58 ============================================== + Hits 5881 5930 +49 - Misses 6910 6919 +9 ```

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[lemunozm]

@mustermeiszer @wischli this is ready to review/merge. The logic itself goes in a nex PR