Improve the feedback provided by krillta proxy signer show-request

[Koenvh1]

This changes the feedback provided by krillta proxy signer show-request to include information about certificate expiration and renewal time:

$ krillta proxy signer show-request
-------------------------------
nonce: 06d7845b-61aa-4ce9-97cf-d7942a1a5a9a
-------------------------------

Certificates will be reissued 80 weeks before expiry.
The current certificate expires on 2039-12-05T12:47:15+00:00.
Ergo the certificate is eligible for renewal on 2038-05-24T12:47:15+00:00.

NOTE: Use the JSON output for the signer.

Additionally, error when the TA and Krill config don't match:

$ krillta signer -c ta.conf --format text process --request request.json
TA timing config between krillta and krill do not match!

This does change the format of those request files (two additional fields), which are now required.

(Separated from #1237 because it changes the format and requires NLnetLabs/rpki-rs#310)