Add optional requirements fields to all templates

[AbrilRBS]

Changelog: (Feature): Add optional requirements fields to all templates
Docs: conan-io/docs#2881

Just as for the basic template, this now lets you add multiple requirements to your templates if needed, such as:
conan new cmake_exe -d name=game -d version=1.0 -d requires=maths/1.3 -d requires=ai/2.0

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[memsharded]

Some test might be failing for changed hashes of contents (you might avoid it by avoiding extra blank lines in the template if requires not defined)

[AbrilRBS]

Oops, I thought I had submitted this as a draft. I knew about the failing tests and wanted to fix it before un-drafting it, but I guess it's too late now for it now that you've reviewed it @memsharded 😄

[memsharded]

I am fine with merging this, and postponing the implementation of real include and linking and executing dependencies for a later PR, and maybe not necessary to do it at once for all templates, but one at a time also good (maybe it would have been better to introduce the new requires in templates one at a time while implementing the whole functionality, but I am fine with it too this way)

[AbrilRBS]

After talking with @czoido and @memsharded, let's rethink how we approach this and merge something minimal, and then create a cmake template that works, and we can then add more interesting stuff such as the toy example suggested by Carlos