plugin-caller: add missing methods from acme.sh

plugin-caller: add missing methods from acme.sh

As reported in our community forum [0] certain dns plugins use code
from `acme.sh`, which is currently not in our proxmox-acme.

I initially only added _sign and it's callees, but then though about
trying to get all missing methods somehow (only resethttp() was
missing in addition).

The heuristic used to get all missing methods was grepping for '\b_'
in all dns plugins and then removing:
* declarations in proxmox_acme (already present)
* methods declared in the plugins themselves
* $_.* (or ${_.*) - variable use
* comments

in shell:
```
present=$(awk 'BEGIN{ORS="|";} /^_/{ gsub(/\(\) {/, ""); print $0}' \
  src/proxmox-acme | | sed -r 's/\|$//')
local=$(awk 'BEGIN{ORS="|";} /^_/{ gsub(/\(\) {/, ""); print $0}' \
  src/acme.sh/dnsapi/dns*.sh | sed -r 's/\|$//')
grep '\b_' src/acme.sh/dnsapi/* | grep -Ev \
  "$present|$local|_[a-zA-Z0-9_-]+=|\\$\{?_|^src/acme.sh/dnsapi/.*sh:#"
```

[0] https://forum.proxmox.com/threads/proxmox-acme-with-transip-plugin-_sign-command-not-found.92582/

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>