Offer also POST, not only GET on /apply for sync flow

[pawel-kow]

Feedback from Q Misell during EITF120 regext session:
In case of bigger payloads in variables (like several DKIM keys) it may happen that the maximum URL length might be exceeded.
For this case a possibility to use POST may be considered.

[pawel-kow]

Note from @pawel-kow:

• this may also improve privacy and/or security if the variables would contain some sensitive information which should not be logged by intermediaries. This is not a big advantage however, as DNS is public by default.