prysmctl support generating non-phase0 genesis.ssz

[kasey]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

Adds a --fork flag to prysmctl testnet generate-genesis to specify other forks beyond phase0:

   --fork value               Name of the BeaconState schema to use in output encoding. [phase0,altair,bellatrix,capella] (default: phase0)

Other notes for review

This was added to help develop changes to allow testnets/e2e to start from a post-merge block. This PR does not unlock that capability, just a small helpful utility broken out into its own PR.

[prestonvanloon]

I ran gazelle, but you have more bazel deps viz issues now.

ERROR: /home/preston/git/prysm/cmd/prysmctl/testnet/BUILD.bazel:3:11: in go_library rule //cmd/prysmctl/testnet:go_default_library: target '//beacon-chain/core/altair:go_default_library' is not visible from target '//cmd/prysmctl/testnet:go_default_library'. Check the visibility declaration of the former target if you think the dependency is legitimate
ERROR: /home/preston/git/prysm/cmd/prysmctl/testnet/BUILD.bazel:3:11: in go_library rule //cmd/prysmctl/testnet:go_default_library: target '//beacon-chain/core/execution:go_default_library' is not visible from target '//cmd/prysmctl/testnet:go_default_library'. Check the visibility declaration of the former target if you think the dependency is legitimate
ERROR: /home/preston/git/prysm/cmd/prysmctl/testnet/BUILD.bazel:3:11: Analysis of target '//cmd/prysmctl/testnet:go_default_library' failed
ERROR: Analysis of target '//cmd/prysmctl:prysmctl' failed; build aborted: 

[prestonvanloon]

I pushed the fix for gazelle viz changes.