fix(grpcHeaders): accept values with "=" symbols

[artburkart]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

Before this commit, it was not possible to pass in base64-encoded
content as a header value if it contained an equals sign. This commit
changes the behavior slightly. Rather than ignore key/value pairs where
the value happens to have an equals sign, we assume the first equals
sign delimits the key from the value and pass in the rest of the value
as-is.

Also, instead of printing the header name along with its value, we only
print the name, so there is less risk of leaking information into logs
that shouldn't be there.

Which issues(s) does this PR fix?

N/A

Other notes for review

This has not been tested in the context of the full validator client.
Instead, a small example was made to demonstrate the feasibility. The
example is shown here:

package main

import (
	"log"
	"os"
	"strings"

	"github.com/davecgh/go-spew/spew"
	"github.com/urfave/cli/v2"
)

type Config struct {
	GrpcHeadersFlag string
}

func main() {
	app := &cli.App{
		Action: func(c *cli.Context) error {
			for _, hdr := range strings.Split(c.String("grpc-headers"), ",") {
				if hdr != "" {
					ss := strings.Split(hdr, "=")
					spew.Dump(ss[0])
					spew.Dump(strings.Join(ss[1:], "="))
				}
			}
			return nil
		},
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name: "grpc-headers",
				Usage: "A comma-separated list of key value pairs to pass as gRPC headers for all gRPC " +
					"calls. Example: --grpc-headers=key=value",
			},
		},
	}

	err := app.Run(os.Args)
	if err != nil {
		log.Fatal(err)
	}
}

Example invocation:

❯ go run main.go --grpc-headers=key=value,Authorization="Basic $(echo -n hello:world | base64)"
(string) (len=3) "key"
(string) (len=5) "value"
(string) (len=13) "Authorization"
(string) (len=22) "Basic aGVsbG86d29ybGQ="

[CLAassistant]

All committers have signed the CLA.

[rkapka]

First of all, thank you for the contribution!

This piece of code was never tested, but the functionality was straightforward. Now that it got more complicated, can you please add a suite of tests for various header values? It will greatly eliminate the risk of introducing bugs in case we need to add more code on top.

[artburkart]

Hi @rkapka,

Do you know if there's any faster way to invoke the tests than this?

$ go test github.com/prysmaticlabs/prysm/validator/client
ok  	github.com/prysmaticlabs/prysm/validator/client	44.623s

If not, is it okay if we pull the snippet of code into something that can be unit tested?

[artburkart]

Ah, it looks like when I run individual tests, it goes way faster.

[artburkart]

We've added 5 new tests cases. 👍

[artburkart]

Ooops, I forgot to add a failure case. One moment please.

[artburkart]

If this is considered too hacky, we can take the time to improve it.

[prestonvanloon]

@artburkart. Please run gazelle. I tried to push to your branch but i dont have access.

bazel run //:gazelle

Requested changes were addressed

[artburkart]

Thank you! 🙏