Add mime type in ExecuteResponse in runner service

[adambabik]

Mime type is sent in the first ExecuteReponse that contains any stdout; it is sent only once.

This implementation is pretty native; it analyses the first read stdout chunk and tries to detect the mime type. If it's not found, then the stdout is sent and mime type is not detected again.

The alternative approach might be to buffer stdout and try to detect mime type multiple times until N bytes is read. If there is no result after N bytes, the detection is stopped. Still, the detected mime type should be sent only one per Execute() invocation, however, it could arrive later than in the first ExecuteResponse.

@sourishkrout My approach would be to test this approach against some real world examples. If we are not satisfied with the result, I can implement the alternative.

Closes #545

[sourishkrout]

Did a number of manual tests with our canonical examples and this works really well already 👍. Let's merge.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
90.9% Coverage on New Code
0.0% Duplication on New Code

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