We intend to delete the v1 API (i.e. `wfe` and its associated codepaths)
in the near future, and as such are not giving it new features or
capabilities. However, before then we intend to allow the v2 API to
provide issuance both from our RSA and from our ECDSA intermediates.
The v1 API cannot gain such capability at the same time.
The CA doesn't know which frontend originated any given issuance
request, so we can't simply gate the single- or double-issuer behavior
based on that. Instead, this change introduces the ability for the
WFE (and the RA, which sits between the WFE and the CA) to request
issuance from a specific intermediate. If the specified intermediate is
not available in the CA, issuance will fail. If no intermediate is
specified (as is the case in requests coming from wfe2), it falls back
to selecting the issuer based on the algorithm of the public key to
be signed.
Fixes#5216
400bf3a02a