Add Goland Standard "cmd" Pattern for Validator Binaries

[rauljordan]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

Based on https://hackmd.io/i_B-BH-NRDmDCIxRAsY6eg, we should align Prysm to more Golang standards, creating a standard cmd entrypoint for binaries.

[prestonvanloon]

Can you alias the previous targets so that this doesn't break any existing workflows?

Edit: I mean all of the targets, images, etc

[prestonvanloon]

These were the previous targets

//validator:push_images_debug
//validator:push_images_alpine
//validator:push_images
//validator:image_bundle_debug
//validator:image_debug
//validator:image_bundle_alpine
//validator:image_bundle
//validator:image_with_creation_time
//validator:image_alpine
//validator:image
//validator:validator
//validator:go_default_test
//validator:go_default_library

[rauljordan]

ok awesome, on it