import_headers takes an array of per-spec option tables, each describing
how a single header name or pattern should be imported into the message
metadata. Compared to import_x_headers it adds:
* Trailing-`*` wildcard patterns (e.g. `X-*`) alongside exact names.
Bare/leading/interior wildcards are rejected at compile time.
* `match` of `first`, `last` (default), or `all`. `all` captures every
matching header instance as an array of strings; the others capture
a string. Specs that produce no matches write nothing.
* `transform` selects the metadata key style: `snake_case` (default,
matches the existing import_x_headers behavior), `kebab_case`,
`camel_case`, or `pascal_case`. Header matching itself is always
case-insensitive.
* `target` overrides the metadata key for exact-name specs.
* `remove` strips the matched headers from the message body in a
single follow-up pass.
When more than one spec could match a header, the first matching spec
wins, so callers can place specific rules ahead of a wildcard catch-all.
import_x_headers now delegates to import_headers, so its behavior is
unchanged and the two share a single implementation.
retain_headers now passes the header index alongside the &Header to its
closure, which import_headers uses for its post-pass removal step
instead of tracking a parallel counter. Existing callers that don't
need the index ignore it with `_`.
Closes: #515
Wez Furlong