Update get-single-profile-picture API to return profile image by username

[brootle]

This update will have dramatic optimization effect on client side apps and on nodes.

Right now in order to get profile image apps have to do get-single-profile API call in order to get user public key because URL to profile image is based on user public key like this

https://bitclout.com/api/v0/get-single-profile-picture/BC1YLiUt3iQG4QY8KHLPXP8LznyFp3k9vFTg2mhhu76d1Uu2WMw9RVY

If we allow https://bitclout.com/api/v0/get-single-profile-picture/brootle apps will not have to make that extra API call to deso nodes.