Prop 456: Establish new Core StableSwap pool (USDC/USDT)

This proposal recognises the StableSwap pool of USDC/USDT as a core StableSwap pool and enables OSMO incentives at the next routine incentives proposal as well as providing initial OSMO incentives to establish functional liquidity.

## Details
This is a proposal to include the StableSwap pool USDC/USDT (#939) into the set of pools that receive OSMO liquidity incentives by recognising it as a core StableSwap pool.

This proposal aims to establish a target liquidity of at least .15m by allocating 0.182% of Liquidity incentives directly and maintaining this for 4 routine incentive proposals.

After this period has expired, the pool will continue to receive incentives as a Stable/Stable pool according to the regular incentives program.

The pool incentivised at this time would be Pool #939 with all assets listed provided via the Axelar bridge and as such are the canonical representations of these on Osmosis according to Proposal 206. If the canonical status of one of these assets is changed then incentives would move at the next routine proposal to the pool containing the new canonical assets, with the highest volume and matching the criteria stated in Proposal 377.

Stable pools are incentivised by the model defined in Proposal 377. In brief, incentivised pools receive a subsidy of up to 2 times the value of fees generated in the pool in OSMO, with the total of the Stable/Stable category being capped as 4% of current Liquidity Provider emissions.

## About USDC
USDC is a stablecoin redeemable on a 1:1 basis for US dollars, backed by dollar-denominated assets held in segregated accounts with US-regulated financial institutions and issued by Circle.

Website: https://www.circle.com/en/usdc

## About USDT
Launched in 2014, Tether was the first stablecoin issued and remains the most prevalent stablecoin in use across the crypto ecosystem.

Website: https://tether.to/

Commonwealth Thread: https://commonwealth.im/osmosis/discussion/10265-establish-new-core-stableswap-pool-usdcusdt