Allow bulk writes

[Al2Klimov]

... to combine several modifications in one request (which seem to be rate-limited btw., at least this lib rate-limits them).

I.e.: https://desec.readthedocs.io/en/latest/dns/rrsets.html#modifying-an-rrset

[adamhassel]

Would that not simply be changing a function signature or two?

Like

func (s *RecordsService) Update(ctx context.Context, domainName, subName, recordType string, rrSet RRSet) (*RRSet, error)

becomes

func (s *RecordsService) Update(ctx context.Context, domainName, subName, recordType string, rrSet ...RRSet) ([]RRSet, error)

and then you update the test and the docs, and Bob's your uncle?