### Summary

`ansible-galaxy collection install` makes a lot of requests to galaxy.ansible.com and can occasionally hit the rate limit. When that happens, galaxy.ansible.com will return a 520 http code and `ansible-galaxy` fails to download the collection and exits. Ideally when the client encounters a rate limiting http code (either 520, or 429), it should wait, slow down the request rate and try again rather than exiting.

More information is available here: https://github.com/ansible/galaxy/issues/2429 

### Issue Type

Bug Report

### Component Name

ansible-galaxy

### Ansible Version

```console
$ ansible --version
root@ubuntu-s-1vcpu-1gb-nyc3-01:~# ansible --version
ansible 2.10.7
  config file = None
  configured module search path = ['/root/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/lib/python3.8/dist-packages/ansible
  executable location = /usr/local/bin/ansible
  python version = 3.8.5 (default, Jan 27 2021, 15:41:15) [GCC 9.3.0]
```


### Configuration

```console
$ ansible-config dump --only-changed
root@ubuntu-s-1vcpu-1gb-nyc3-01:~# ansible-config dump --only-changed
root@ubuntu-s-1vcpu-1gb-nyc3-01:~#
```


### OS / Environment

All

### Steps to Reproduce

Run `ansible-galaxy collection install `amazon.aws`. If your internet is fast enough, this will occasionally fail when galaxy.ansible.com returns a 520 error code.

### Expected Results

Collection should be installed.

### Actual Results

```console
`ansible-galaxy` encounters a 429 or 520 http code and exits.
```


### Code of Conduct

I agree to follow the Ansible Code of Conduct


token = KeycloakToken(auth_url='https://api.test/')
    mock_token_get = MagicMock()
    mock_token_get.return_value = 'my_token'
    monkeypatch.setattr(token, 'get', mock_token_get)
    api = GalaxyAPI(None, "test", "https://galaxy.ansible.com/api/", token=token)
    actual = {}
    api._add_auth_token(actual, "https://galaxy.ansible.com/api/v3/resource/name", required=True)
    assert actual == {'Authorization': 'Bearer my_token'}
def test_api_token_auth_with_v2_url():
    token = GalaxyToken(token=u"my_token")
    api = GalaxyAPI(None, "test", "https://galaxy.ansible.com/api/", token=token)
    actual = {}
    # Add v3 to random part of URL but response should only see the v2 as the full URI path segment.
    api._add_auth_token(actual, "https://galaxy.ansible.com/api/v2/resourcev3/name", required=True)
    assert actual == {'Authorization': 'Token my_token'}
def test_api_basic_auth_password():
    token = BasicAuthToken(username=u"user", password=u"pass")
    api = GalaxyAPI(None, "test", "https://galaxy.ansible.com/api/", token=token)
    actual = {}
    api._add_auth_token(actual, "", required=True)
    assert actual == {'Authorization': 'Basic dXNlcjpwYXNz'}
def test_api_basic_auth_no_password():
    token = BasicAuthToken(username=u"user")
    api = GalaxyAPI(None, "test", "https://galaxy.ansible.com/api/", token=token)
    actual = {}
    api._add_auth_token(actual, "", required=True)
    assert actual == {'Authorization': 'Basic dXNlcjo='}
def test_api_dont_override_auth_header():