Baggage creation for head of trace

[sl0thentr0py]

closes #1587, #1479

[antonpirker]

Just some nitpicking.
Your change is easy readable like the "Kronenzeitung". I like it! (the change, not the newpaper)

[sl0thentr0py]

testing with sample app where Django renders an html template, and then the JS bundle with react runs, so python is the head SDK.

DSC on python transaction
{'trace_id': '81c052f3d99f42d781598e36a525788e', 'public_key': 'd655584d05f14c58b86e9034aab6817f', 'sample_rate': '1.0', 'release': '0.0.1', 'environment': 'dev', 'transaction': '/server_side/app/'}

meta tag injected into HTML
<meta name="sentry-trace" content="81c052f3d99f42d781598e36a525788e-b6635497ec50569a-1">
<meta name="baggage" content="sentry-trace_id=81c052f3d99f42d781598e36a525788e,sentry-environment=dev,sentry-release=0.0.1,sentry-public_key=d655584d05f14c58b86e9034aab6817f,sentry-transaction=/server_side/app/,sentry-sample_rate=1.0">


DSC on JS transaction
{"trace_id":"81c052f3d99f42d781598e36a525788e","environment":"dev","release":"0.0.1","public_key":"d655584d05f14c58b86e9034aab6817f","transaction":"/server_side/app/","sample_rate":"1.0"}