anthropic: Allow specifying additional beta headers for custom models (#20551)

Release Notes:

- Added the ability to specify additional beta headers for custom
Anthropic models.

---------

Co-authored-by: David Soria Parra <167242713+dsp-ant@users.noreply.github.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/language_models/src/settings.rs

@@ -97,6 +97,7 @@ impl AnthropicSettingsContent {
                                     cache_configuration,
                                     max_output_tokens,
                                     default_temperature,
+                                    extra_beta_headers,
                                 } => Some(provider::anthropic::AvailableModel {
                                     name,
                                     display_name,
@@ -111,6 +112,7 @@ impl AnthropicSettingsContent {
                                     ),
                                     max_output_tokens,
                                     default_temperature,
+                                    extra_beta_headers,
                                 }),
                                 _ => None,
                             })