[Feature Request] power related convex functions

Hi.

CVX supports power-related functions:
* pow_abs(x,p) = |x|^p
* pow_pos(x,p) = max{x,0}^p
* pow_p(x,p)
http://cvxr.com/cvx/doc/funcref.html

These functions are often used for financial problems, like portfolio optimization.

I think these functions are important and useful, but current Convex.jl haven't supported them yet.
https://convexjl.readthedocs.io/en/latest/operations.html

I request to support the functions.

Start by comparing the requested pow_abs, pow_pos, and pow_p definitions in the linked CVX function reference with Convex.jl's operations documentation. Identify the relevant operation entry points and existing tests, then determine the supported domains, convexity rules, and solver behavior needed for the feature. Done means the functions are documented and covered by tests.

Written by the indexing model from the issue text.