otel,loki,bigquery: add headers() option to dest

[alltilla]

Example config:

opentelemetry(
  ...
  headers(
    "organization" => "Axoflow"
    "stream-name" => "axo-stream"
  )
);

[github-actions]

This Pull Request introduces config grammar changes

axoflow/f18641e31f1a2884b4a36c731e60cfc0d5bace62 -> alltilla/otel-dest-headers

--- a/destination
+++ b/destination

 axosyslog-otlp(
+    headers(
+        <empty>
+        <string> => <string>
+    )
 )

 bigquery(
+    headers(
+        <empty>
+        <string> => <string>
+    )
 )

 loki(
+    headers(
+        <empty>
+        <string> => <string>
+    )
 )

 opentelemetry(
+    headers(
+        <empty>
+        <string> => <string>
+    )
 )

 syslog-ng-otlp(
+    headers(
+        <empty>
+        <string> => <string>
+    )
 )

[bazsi]

I tested the headers() option for Otel as I had a good enough test environment locally and it seems to work fine. It did lowercase the header as expected and the tcpdump capture did contain the header added.