Refactor dependencies, make Prysm "go gettable"

[prestonvanloon]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

At Prysmatic Labs, we use the Bazel build tool for many reasons including:

• Reproducible builds
• Polyglot language support
• Reliable build configuration / customization
• Powerful query language
• Cross compilation support
• Docker build support
• And many more (full documentation page coming soon FR: Why Bazel? documentation#138)

However, we recognize that Bazel build tooling adds friction to the average go developer. As such, we would like to provide first class support for go build/test. Additionally, for those users that wish to build upon Prysm with bazel, we're providing a prysm_deps() starlark rule that can be loaded in the downstream WORKSPACE.

# In your WORKSPACE file.

http_archive(
    name = "prysm",
    strip_prefix = "prysm-master",
    urls = [ # Ideally, choose a specific commit instead of using master.
        "https://github.com/prysmaticlabs/prysm/archive/master.tar.gz",
    ],
)

load("@prysm//:deps.bzl", "prysm_deps") 
prysm_deps()

Which issues(s) does this PR fix?

Fixes #6035 (go build)
Fixes #6005 (deps refactor for downstream users)
Fixes #6011 (update deps to recent version)

Other notes for review

• Refactor deps to be loadable by downstream users (See deps.bzl)
• Add go build/test CI rule (using github actions)
• Add documentation on dependency updates and policy (see DEPENDENCIES.md)
• CI green checkmark

** Diff is large due to generated files. **

Please review:

• WORKSPACE
• DEPENDENCIES.md
• deps.bzl
• Changes in go files

Other notable changes:

• This PR also cherry-picks 82f46cc (Update Gossipsub to 1.1 #5998) from v0.12.
• This PR includes a patch for libp2p TLS to disable S390X, which was unused. See crypto.go:217:20: undefined: cpu.S390X libp2p/go-libp2p-tls#66.
• This PR skip two tests (see 718dd3e), which are fixed, refactored, or obsolete in v0.12 updates (Update to v0.12 #5614).
• I included update rules_go to v0.23.0 and gazelle to v0.21.0 #5900 but reverted it since it was causing some other issues. We'll have to attempt updating rules_go and gazelle in a follow up PR.

[nisdas]

This looks great

[alonmuroch]

@prestonvanloon this is awesome!
Will change our integration from bazel to go mod and will report back :)