ra/sa: render the attempted challenge as "processing" - #9096

RFC 8555 §7.1.6 says a challenge moves from "pending" to "processing" once the
client responds to it. Boulder never rendered that state: challenges are stored
as part of the authorization rather than as separate objects, so a challenge
stayed "pending" until the authz was finalized as "valid" or "invalid".

#8838 added SetAuthzProcessing, which sets authz2.beganProcessing before
validation is dispatched, but that column has no effect on rendered objects.
Following @aarongable's suggestion in #9032, this extends it so the RA also
tells the SA which challenge is being validated, and the SA renders that
challenge as "processing" while the authz stays "pending":

• SetAuthzProcessing now takes a dedicated SetAuthzProcessingRequest
  carrying the authz ID and the attempted challenge type. Field 1 is unchanged
  from AuthorizationID2, so the RPC stays wire-compatible in both directions
  during a deploy; the new field is optional and an RA that doesn't send it
  gets the old behaviour.
• The SA writes the challenge type into the attempted column in the same
  UPDATE that sets beganProcessing.
• modelToAuthzPB renders an attempted challenge on a still-pending authz as
  "processing" (with no validation results). The authz status stays "pending",
  since RFC 8555 has no "processing" status for authorizations. The
  combination is done in the SA rather than the WFE because beganProcessing
  isn't part of the authz model/proto, and attempted != NULL on a pending
  authz is only ever produced by SetAuthzProcessing.
• The RA rejects a second attempt at a challenge that is already "processing"
  with the same "conflict" error the SA returns, so clients that re-POST keep
  getting the 409 introduced by ra/sa: Prevent parallel validation attempts #8838 (and TestParallelValidationConflict
  keeps passing) instead of a "malformed" 400 from CheckPending.
• test/inmem/sa now forwards SetAuthzProcessing; it didn't before, so RA
  tests couldn't exercise the flag.

Everything is still gated behind the existing SetAuthzProcessing feature
flag. No config, schema, or flag changes.

Open questions for reviewers:

• Once a challenge is attempted, sibling challenges are hidden (existing
  behaviour). While processing they're shown the same way; happy to render
  them as "pending" instead if you'd rather follow §7.1.4 literally.
• The conflict error text is duplicated between RA and SA; can hoist to a
  shared constant if preferred.

Tests: new TestAuthzModelProcessing, extended TestSetAuthzProcessing,
TestPerformValidationSetsChallengeProcessing,
TestPerformValidationChallengeProcessing, and WFE challenge/authz handler
cases asserting the rendered "processing" status. Protos regenerated with the
pinned toolchain (protoc 3.20.1, protoc-gen-go v1.36.5, protoc-gen-go-grpc
v1.5.1).

Fixes #9032