Build trace request parameters should not be base64-encoded entirely

In gitlab-com/infrastructure#2888, we found that the logging for the build trace was generating unique keys for every build trace input because of URLs such as:

Instead of the giant blob being base64-encoded, shouldn't we be encoding each key-value pair as such? For example:

state=eyJvZmZzZXQiOjE2MzQsIm5fb3Blbl90YWdzIjowLCJmZ19jb2xvciI6bnVsbCwiYmdfY29sb3IiOm51bGwsInN0eWxlX21hc2siOjB9

/cc: @filipa