pkg/util/log: HTTP log sink has inefficiencies

**Is your feature request related to a problem? Please describe.**

- Incorrect allocation of dynamic headers map: it derives from [FileBasedHeaders](https://github.com/cockroachdb/cockroach/blob/588257ea305973292821d38622ad84ed1df83996/pkg/util/log/http_sink.go#L67) but its size is based on [allocated from Headers](https://github.com/cockroachdb/cockroach/blob/588257ea305973292821d38622ad84ed1df83996/pkg/util/log/http_sink.go#L63).

- Custom headers are not added to GET requests. (this might be a feature instead of a bug but still don't understand the reasoning behind it). [source](https://github.com/cockroachdb/cockroach/blob/588257ea305973292821d38622ad84ed1df83996/pkg/util/log/http_sink.go#L178)

- A new gzip writer is being created for every request instead of reusing it: [source](https://github.com/cockroachdb/cockroach/blob/588257ea305973292821d38622ad84ed1df83996/pkg/util/log/http_sink.go#L132).

- Repeated creation of requests and modification instead of cloning them.

Jira issue: CRDB-53074