The PR follows https://github.com/StackExchange/dnscontrol/pull/3542
Found some bugs when running intergration tests locally again, and the
PR is an attempt to fix them:
- When updating/creating HTTPS/SRV records, Vercel API only reads from
the corresponding struct (either `srv` or `https`). If we provide a
`value`, the Vercel API will reject with an error.
- The PR makes `Value` "nil-able", and sets `Value` to nil when dealing
with `SRV` or `HTTPS` records.
- When updating a record, currently, we treat the empty SVC param as
omitting the field. But with Vercel's API, omitting a field means not
updating the field. We need to explicitly make the field an empty string
to create/update an empty SVC param, and the PR does that.
- Vercel implements an unknown `ech=` parameter validation process for
HTTPS records. The validation process is unknown, undocumented, thus I
can't implement a `rejectif` for `AuditRecord`.
- Let's make this a known caveat, describe it in the provider docs, skip
these intergration tests, and move on.
Please tag this PR w/ `provider-VERCEL`.
