lib: add `bufq` and `dynhds`

lib: add `bufq` and `dynhds`

Adding `bufq`:
- at init() time configured to hold up to `n` chunks of `m` bytes each.
- various methods for reading from and writing to it.
- `peek` support to get access to buffered data without copy
- `pass` support to allow buffer flushing on write if it becomes full
- use case: IO buffers for dynamic reads and writes that do not blow up
- distinct from `dynbuf` in that:
  - it maintains a read position
  - writes on a full bufq return CURLE_AGAIN instead of nuking itself
- Init options:
  - SOFT_LIMIT: allow writes into a full bufq
  - NO_SPARES: free empty chunks right away
- a `bufc_pool` that can keep a number of spare chunks to
  be shared between different `bufq` instances

Adding `dynhds`:
- a straightforward list of name+value pairs as used for HTTP headers
- headers can be appended dynamically
- headers can be removed again
- headers can be replaced
- headers can be looked up
- http/1.1 formatting into a `dynbuf`
- configured at init() with limits on header counts and total string
  sizes
- use case: pass a HTTP request or response around without being version
  specific
- express a HTTP request without a curl easy handle (used in h2 proxy
  tunnels)
- future extension possibilities:
  - conversions of `dynhds` to nghttp2/nghttp3 name+value arrays

Closes #10720