Update headers hostcalls to adaptive buffers and deprecate remaining limits APIs

[joeshaw]

The hostcalls that deal with headers (names & values) are the last ones that aren't set up for adaptive buffers. This means that users need to use the limits API to increase from the default 8k size if they expect more than that. The Rust SDK previously had something similar but we decided to eliminate the separate Compute-specific limit and instead follow only the platform limits.

We should do the same in the Go SDK, and deprecate the SetMaxHeaderNameLen and SetMaxHeaderValueLen calls. The other calls in that struct have already been deprecated with the move to adaptive buffers.