Function fetchSysvarEpochRewards

Fetch the EpochRewards sysvar.

Tracks the progress of epoch rewards distribution. It includes:

• Total rewards for the current epoch, in lamports.
• Rewards for the current epoch distributed so far, in lamports.
• Distribution completed block height, i.e. distribution of all staking rewards for the current epoch will be completed at this block height.

Note that EpochRewards only lasts for a handful of blocks at the start of an epoch. When all rewards have been distributed, the sysvar is deleted. See https://github.com/solana-labs/solana/blob/e0203f22dc83cb792fa97f91dbe6e924cbd08af1/docs/src/runtime/sysvars.md?plain=1#L155-L168