Document externally managed rustls crypto providers for compliance-sensitive applications

## Problem

The Rust crates already support externally managed rustls crypto providers by disabling `default-rustls-provider` and installing a provider in the application.

That capability is present today, but it is easy to miss when integrating the libraries into compliance-sensitive applications that need centralized control over rustls provider selection.

## Request

Please add a guide/example that shows:

1. Using the default provider path.
2. Disabling the default provider features in Google Cloud crates.
3. Installing a rustls crypto provider explicitly in the application via `rustls::crypto::CryptoProvider::install_default()`.
4. A short note that this improves crypto-provider control but is not, by itself, a compliance guarantee.

## Why this helps

This would give users a concrete, supported path for:

- centralized provider installation at application startup
- custom provider selection
- auditability in stricter environments

I have a docs-only PR prepared for this if the direction sounds good.