Feature-select OTLP exporter TLS backend

[seanarnold]

What was changed

PR #1274 made the TLS crypto backend
feature-selectable (tls-ring / tls-aws-lc) for the gRPC service client. This extends
that to the OTLP metric exporter. Previously opentelemetry-otlp hardcoded tls-roots +
reqwest-rustls, which always pulled in ring. Those are removed and replaced with
feature-gated wiring (tls-ring keeps today's behavior; tls-aws-lc routes the exporter
through aws-lc-rs), using opentelemetry-otlp 0.31.1's own tls-ring/tls-aws-lc
features.

Why?

#1274 covered the gRPC path, but the OTLP exporter still hardcoded ring, so a
ring-free, aws-lc-rs-only build (required for FIPS-capable deployments) wasn't
possible. Building with --no-default-features --features tls-aws-lc,otel now yields a
dependency tree free of ring.

Checklist

1. Closes (no issue)

2. How was this tested:

   • cargo tree -i ring across all targets: empty under tls-aws-lc, still present
     under the default tls-ring (no change to default builds).
   • cargo check/clippy clean for common (both backends) and sdk-core (default
     and aws-lc feature sets).

3. Any docs updates needed?

   CHANGELOG.md updated.

[CLAassistant]

All committers have signed the CLA.

[seanarnold]

@Sushisource Are you able to take a look at this one please? 🙇

[Sushisource]

Thanks!

[seanarnold]

Thanks @Sushisource. Do you think you could cut a new release, since there's quite a few changes since the last one v0.4.0...main

We're looking to use https://github.com/temporalio/sdk-ruby/ in a FIPS environment so would appreciate a release so we can follow up with a PR against that gem.

[Sushisource]

@chris-olszewski Can comment on making a release. I think there's a few things he wants to get in first but I doubt it'll be long.

[chris-olszewski]

@seanarnold Are you waiting for this to make it into the Ruby SDK? In the Ruby SDK we depend on the Rust core via a Git submodule so we don't need a tagged release of the Rust crates.