feat: implement multi header support

[theseion]

• support multiple headers with the same name

• support multiple headers with identical name and value

• make list of headers ordered; in the future, tests should be able to enforce the order of headers in a request

• improve API, tests, and documentation of ftwhttp.Header

• disable logging in tests where possible

• enable self-updater test (go-critic was complaining because the test file was touched)

Fixes #332

[theseion]

Note that our test format currently does not support multiple headers with the same name, because we use a map for headers. In the future we should use a list instead in order to allow for repeating headers and values.

[fzipi]

Thanks for taking this. Big change.

I'll suggest you to split in 3 PRs, as we are mixing stuff here.

1. All the zerolog changes (🙏 good catch). E.g. inclusion and initialization.

func (s *responseTestSuite) SetupSuite() {
	zerolog.SetGlobalLevel(zerolog.Disabled)
}

2. The uncommented self_updater tests
3. This PR, rebased after the above.

It will make things more clear if we need to revert something.

[fzipi]

Looks like tests are broken?

[theseion]

Extended PR with updated schema and support for new ordered_headers field.