PORKBUN: support CAA

[imlonghao]

close #3199

Testes passed

CAA with whitespace not supported.

=== RUN   TestDNSProviders/example.com/33:CAA:CAA_whitespace
    integration_test.go:248:
        ± MODIFY example.com CAA (128 issuewild ";" ttl=600) -> (0 issue "letsencrypt.org; validationmethods=dns-01; accounturi=https://acme-v02.api.letsencrypt.org/acme/acct/1234" ttl=600), porkbun ID: 437665127
    integration_test.go:269: Expected 0 corrections on second run, but found 1.
    integration_test.go:271: UNEXPECTED #0: ± MODIFY example.com CAA (0 issue "letsencrypt.org;" ttl=600) -> (0 issue "letsencrypt.org; validationmethods=dns-01; accounturi=https://acme-v02.api.letsencrypt.org/acme/acct/1234" ttl=600), porkbun ID: 437665127

[tlimoncelli]

I love when a new feature can be added so simply!

Please run go generate and commit the new documentation/providers.md file.

Thanks!

[imlonghao]

done.