[3.x] Add custom headers to requests

[DarronEngelbrecht]

Cherry picked commits from #22 for Pest 3.x

This PR allows adding custom headers to the request through the headers method or PEST_STRESS_TEST_OPTIONS variable. We needed this for authentication.

You can use it like this...

$result = stress('example.com')->headers([
    'Authorization' => 'Bearer '. $this->bearerToken,
])

[ihorchepurnyi]

Thanks a lot for this PR! It will be a great update for k6.

[ihorchepurnyi]

Hey @DarronEngelbrecht!

Could you please clarify if you expect to merge this PR this week?

[DarronEngelbrecht]

@ihorchepurnyi - Unfortunately unable to merge this myself perhaps @devajmeireles can advise?

Likely waiting on @nunomaduro to merge?

[ihorchepurnyi]

Hey guys, any updates on this PR?

[nunomaduro]

@DarronEngelbrecht can you PR to the docs this update?

[DarronEngelbrecht]

@nunomaduro - Done, pestphp/docs#301