Only allow one auth method on the server bindings (#589)

## 🎟️ Tracking

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point to where this change is coming from. -->

## 📔 Objective

The server bindings template iterates over all auth methods of all the
servers and generates code for each of them. This means that if the
schema has two servers defined (like the US and EU servers) it will
generate code like this:
```rust
        if let Some(ref token) = config.oauth_access_token {
            request = request.bearer_auth(token.to_owned());
        };
        // This one is just duplicate
        if let Some(ref token) = config.oauth_access_token {
            request = request.bearer_auth(token.to_owned());
        };
```

This won't break anything and is safe to do, but it's kind of wasteful.
Instead, we can just use the first auth method only.

The change doesn't have any effect on the bindings for the moment as
we're only ever defining one server and one auth method, but
bitwarden/server#6653 will add the US and EU
servers to the server API schema, which will result in the above shown
duplication.

## 🚨 Breaking Changes

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## ⏰ Reminders before review

- Contributor guidelines followed
- All formatters and local linters executed and passed
- Written new unit and / or integration tests where applicable
- Protected functional changes with optionality (feature flags)
- Used internationalization (i18n) for all UI strings
- CI builds passed
- Communicated to DevOps any deployment requirements
- Updated any necessary documentation (Confluence, contributing docs) or
informed the documentation
  team

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  issue and could potentially benefit from discussion
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Author: dani-garcia

support/openapi-template/reqwest-trait/api.mustache

@@ -353,6 +353,7 @@ impl {{classname}} for {{classname}}Client {
         {{/hasHeaderParams}}
         {{#hasAuthMethods}}
         {{#authMethods}}
+        {{#-first}}
         {{#supportTokenSource}}
         // Obtain a token from source provider.
         // Tokens can be Id or access tokens depending on the provider type and configuration.
@@ -391,6 +392,7 @@ impl {{classname}} for {{classname}}Client {
         };
         {{/isOAuth}}
         {{/supportTokenSource}}
+        {{/-first}}
         {{/authMethods}}
         {{/hasAuthMethods}}
         {{#isMultipart}}