Add online.net DNS API

[Augustin-FL]

This Pull Request add support for the Online (https://online.net/) DNS API.

Related issue : #2093

[Neilpang]

please confirm whether you have checked EVERY item in this guide : https://github.com/Neilpang/acme.sh/wiki/DNS-API-Dev-Guide

Thanks.

[Augustin-FL]

@Neilpang yes, I have checked every item in this guide.

1. use cloudflare as reference : yes. OVH has also been used as reference
2. script name having the correct name: yes
3. location in dnsapi/ : yes
4. shebang : use sh from env, not something else : yes
5. has dns_myapi_<add|rm> functions: yes
6. add function guides:
   6.1. gets fulldomain/txtvalue in dns_myapi_add(): yes
   6.2. save credentials in _add() : yes, via _online_check_config()
   6.3. detect root zone: yes, via _get_root(). I have tested
   example.org
subdomain.example.org
subsubdomain.subdomain.example.org
domainthatidontown.org
invaliddomain
   6.4. use _post/ _get, not wget / curl : yes, in _online_rest() function.
   6.5 process strings using provided functions and some basic commands (cut,...), not awk: yes.
7. rm function runs standalone: yes, records are correctly removed.
8. support wildcard txt certs: yes, tested with *.example.org
9. updated docs accordingly : yes
10. creating issue related to this DNS provider : yes

[Neilpang]

Please confirm this is ready to merge, and this is still working as expected.

Thanks.

[Augustin-FL]

I confirm that it is still working as expected (I performed tests again to make sure)

@Neilpang this PR is ready to be merged !