### 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


""" Loads the cache file requested if possible. The file must not be world writable. """
    cache_version = 1
    if not os.path.isfile(b_cache_path):
        display.vvvv("Creating Galaxy API response cache file at '%s'" % to_text(b_cache_path))
        with open(b_cache_path, 'w'):
            os.chmod(b_cache_path, 0o600)
    cache_mode = os.stat(b_cache_path).st_mode
    if cache_mode & stat.S_IWOTH:
        display.warning("Galaxy cache has world writable access (%s), ignoring it as a cache source."
                        % to_text(b_cache_path))
        return
    with open(b_cache_path, mode='rb') as fd:
        json_val = to_text(fd.read(), errors='surrogate_or_strict')
    try:
        cache = json.loads(json_val)
    except ValueError:
        cache = None
    if not isinstance(cache, dict) or cache.get('version', None) != cache_version:
        display.vvvv("Galaxy cache file at '%s' has an invalid version, clearing" % to_text(b_cache_path))
        cache = {'version': cache_version}
        
        with open(b_cache_path, mode='wb') as fd:
            fd.write(to_bytes(json.dumps(cache), errors='surrogate_or_strict'))
    return cache
def _urljoin(*args):
    return '/'.join(to_native(a, errors='surrogate_or_strict').strip('/') for a in args + ('',) if a)
class GalaxyError(AnsibleError):