[4.x] Added content search

[eusonlito]

After multiple rejected attempts to add a search on content, here is my PR.

In any monitoring system, search is an essential part, hence the insistence on including it as a feature of Telescope.

This PR keeps the original tag search functionality and adds an additional content search simply by using quotation marks: "text to search".

I really consider very very necessary the search inside Telescope, if not with this PR, please, with a native team development.

Thank you very much :)

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.