Feat/move lp symbol to common package

[nseguias]

Description and Motivation

This PR moves the LP_SYMBOL constant in state to the lp_common.rs package so that other contracts can use it.

Related Issues

316

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Checklist:

• [x] I have read Migaloo's contribution guidelines.
• [x] My pull request has a sound title and description (not something vague like Update index.md)
• [x] All existing and new tests are passing.
• [x] I updated/added relevant documentation.
• [x] The code is formatted properly cargo fmt --all --.
• [x] Clippy doesn't report any issues cargo clippy -- -D warnings.
• [x] I have regenerated the schemas if needed cargo schema.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.50%. Comparing base (f1fdf5b) to head (1aec306). Report is 1 commits behind head on release/v2_contracts.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/v2_contracts #322 +/- ## ===================================================== Coverage 88.50% 88.50% ===================================================== Files 252 252 Lines 23602 23602 ===================================================== Hits 20888 20888 Misses 2714 2714 ```

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