Q:

Template Jenkinsfile for all project

For example:
I have 30 multibranch Git projects. All projects have their own Jenkinsfile. And suddenly I find a cool Jenkins plugin which I want to add for all projects. This is pain to do it on all projects and this is a big waste of time.
Is it possible to create somethink like template Jenkinsfile which will be something like wrapper for project Jenkinsfile or somethink like that which gives me possibility to do changes in 1 place instead of 30 places?
What I want is something like that:
stage {
 ...}
timestamps {
    <include rest of stages definied in projects>
}
stage {
... }

Template file which is in some repo is looked for like this. All of the projects have their own stages defined which are included in the middle of the template Jenkinsfile and defined in project.
So Jenkinsfile in project must:

load template from repository
put stages in the middle of template Jenkinsfile

A:

You could use:

the Config File Provider Plugin using the configuration files in Jenkins Pipelines:
node {
  ...
  configFileProvider(
    [configFile(fileId: 'jenkinsfile-template', ...)]) {
      ...
    }
  ...
}

load: Evaluate a Groovy source file into the Pipeline script:

Takes a filename in the workspace and runs it as Groovy source text.

Shared Libraries:

Oftentimes it is useful to share parts of Pipelines between various projects to reduce redundancies and keep code "DRY".
Pipeline has support for creating "Shared Libraries" which can be defined in external source control repositories and loaded into existing Pipelines.

A:

As I always say,

Friends don't let friends template code.

It's generally my last resort to write code that in turn generates more code. Further, it's not useful in your use case.
What you're effectively asking is, "I have code I want to share between 30 Jenkinsfiles". The best way to share code is, of course, to create a shared library and use that from the code.
Fortunately, this concept is fully supported in Jenkinsfiles. I highly recommend its use for your use case. From the introduction paragraph in the link given above:

As Pipeline is adopted for more and more projects in an organization,
  common patterns are likely to emerge. Oftentimes it is useful to share
  parts of Pipelines between various projects to reduce redundancies and
  keep code "DRY".
Pipeline has support for creating "Shared Libraries" which can be
  defined in external source control repositories and loaded into
  existing Pipelines.

Then, simply update the library and all the Jenkinsfiles which use it will be updated too. You can version your shared library, and the set up will be much easier to debug.

