terraform-provider-databricks Add `http_headers` and `http_path_prefix` provider configuration options for HTTP proxy support

Added two new provider configuration options to support HTTP proxy scenarios where custom headers or URL path rewriting are required:

http_headers - A map of custom HTTP headers added to all API requests (marked as sensitive)
http_path_prefix - A path prefix prepended to all API request URLs

Changes:

Core Implementation (internal/providers/client/databricks_client.go):
- Added HTTPConfig struct to hold custom headers and path prefix
- Added httpTransportWrapper implementing http.RoundTripper that injects headers and prepends path prefix
- Modified PrepareDatabricksClient to accept optional *HTTPConfig and configure HTTP transport
SDKv2 Provider (internal/providers/sdkv2/sdkv2.go):
- Added http_headers schema field (TypeMap, optional, sensitive)
- Added http_path_prefix schema field (TypeString, optional)
- Updated ConfigureDatabricksClient to read and pass HTTP config
Plugin Framework Provider (internal/providers/pluginfw/pluginfw.go):
- Added http_headers schema field (MapAttribute with StringType)
- Added http_path_prefix schema field (StringAttribute)
- Updated configureDatabricksClient to read and pass HTTP config
Unit Tests:
- internal/providers/client/databricks_client_test.go: Tests for httpTransportWrapper RoundTrip behavior
- internal/providers/sdkv2/sdkv2_test.go: Schema validation and configuration tests

Implementation Details:

Uses Go SDK's HTTPTransport field to inject a custom http.RoundTripper
Headers are set on each request via req.Header.Set()
Path prefix is prepended to req.URL.Path (trailing slashes are trimmed)
Both SDKv2 and Plugin Framework providers share the same underlying client configuration
http_headers is marked as sensitive to avoid logging credentials

Example Usage:

provider "databricks" {
  host  = "https://proxy.example.com"
  token = "your-token"

  http_headers = {
    "X-Custom-Auth" = "auth-value"
  }

  http_path_prefix = "/proxy"
}