Put cw-vesting staking features behind a feature flag

[NoahSaso]

Neutron does not support the staking module, and the cw-vesting contract attempts to execute a staking-related message on instantiate for the native token. This means that the vesting contract does not currently work for NTRN on Neutron.

This PR adds a staking feature flag to cw-vesting that explicitly enables the execution variants that have to do with staking, in addition to the withdraw address message on instantiate.

[codecov[bot]]

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```diff @@ Coverage Diff @@ ## development #800 +/- ## ============================================== Coverage ? 96.17% ============================================== Files ? 207 Lines ? 53401 Branches ? 0 ============================================== Hits ? 51359 Misses ? 2042 Partials ? 0 ```

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