Rename ACME certificate verification response proto field from Token to Authorization

[coderabbitai]

Problem

In the ACME certificate verification response proto, we currently have a field named Token that actually contains the ACME key authorization value, not the raw token. This creates semantic confusion.

Current Implementation

In go/apps/ctrl/services/acme/certificate_verification.go:

res.Msg.Token = challenge.Authorization

The field name Token is misleading because we're actually returning the ACME key authorization.

Proposed Solution

Rename the proto field from Token to Authorization to accurately reflect its contents.

Changes needed:

1. Update the proto definition to rename the field
2. Update the Go code to use the new field name
3. Update any callers/tests that reference this field

Context

• PR: fix: get db schemas ready for demo #3878
• Comment: fix: get db schemas ready for demo #3878 (comment)
• Confirmed by: @Flo4604 that "Authorization should be fine"

ACME Specification Context

According to the ACME HTTP-01 challenge specification, the server should serve the key authorization (which is token + "." + account_key_thumbprint), not just the raw token.

[linear]

ENG-2046 Rename ACME certificate verification response proto field from Token to Authorization