fix(iceberg-catalog-rest): set application/x-www-form-urlencoded content-type header for oauth2 request (#1393)

## Which issue does this PR close?

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PR solves error when trying to authenticate through oauth2 endpoint.

This issue appeared since `reqwest` changed it's behavior with this
[PR](seanmonstar/reqwest#2527).

```
Error: Unexpected, context: { code: 400 Bad Request, operation: auth, url: http://127.0.0.1:8080/realms/iceberg/protocol/openid-connect/token, json: {"error":"invalid_request","error_description":"Missing form parameter: grant_type"} } => Received unexpected response, source: invalid type: string "invalid_request", expected struct ErrorModel at line 1 column 26

Caused by:
    invalid type: string "invalid_request", expected struct ErrorModel at line 1 column 26
```

- Closes #.

## What changes are included in this PR?

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Provide a summary of the modifications in this PR. List the main changes
such as new features, bug fixes, refactoring, or any other updates.
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- override `content-type` header to `application/x-www-form-urlencoded`
for oauth2 authenticate requests

## Are these changes tested?

yes:
- tested it manually (through `keycloak`)
- existing tests in repo

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Co-authored-by: Roman Shanin <rshanin@bhft.com>

Author: Erigara

crates/catalog/rest/src/client.rs

@@ -158,12 +158,18 @@ impl HttpClient {
                 .map(|(k, v)| (k.as_str(), v.as_str())),
         );
 
-        let auth_req = self
+        let mut auth_req = self
             .request(Method::POST, &self.token_endpoint)
             .form(&params)
             .build()?;
+        // extra headers add content-type application/json header it's necessary to override it with proper type
+        // note that form call doesn't add content-type header if already present
+        auth_req.headers_mut().insert(
+            http::header::CONTENT_TYPE,
+            http::HeaderValue::from_static("application/x-www-form-urlencoded"),
+        );
         let auth_url = auth_req.url().clone();
-        let auth_resp = self.execute(auth_req).await?;
+        let auth_resp = self.client.execute(auth_req).await?;
 
         let auth_res: TokenResponse = if auth_resp.status() == StatusCode::OK {
             let text = auth_resp