[Feature] Allow customization of welcome text

[fyears]

Is your feature request related to a problem? Please describe.

Hi, I would like to deploy the robot to my source-available project (not "Open Source" defined by OSI).

Currently the robot welcomes new contributors with the text "Like many open-source projects, we ask ...".

Although the welcome text is still kind of valid, but I would like to change it to something like "like many source-available projects, ...", to avoid any potential confusions.

Describe the solution you'd like

Add a optional input to cla.yml,

...
welcome-text: Like many open-source projects, we ask that you sign our $cla-link, before we can ...
...

Additional context

Obviously the text is defined at

github-action/src/pullrequest/pullRequestCommentContent.ts

Line 31 in dbc1c64

let lineOne = (input.getCustomNotSignedPrComment() || `<br/>Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that $you sign our [Developer Certificate of Origin](${input.getPathToDocument()}) before we can accept your contribution. You can sign the DCO by just posting a Pull Request Comment same as the below format.<br/>`).replace('$you', you)

and

github-action/src/pullrequest/pullRequestCommentContent.ts

Line 73 in dbc1c64

let lineOne = (input.getCustomNotSignedPrComment() || `<br/>Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that $you sign our [Contributor License Agreement](${input.getPathToDocument()}) before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.<br/>`).replace('$you', you)

. We need to pass a optional variable to those location.

I am not familiar with github action development so I'm afraid I don't know how to fix this even I would like to make a PR. :-(

[fyears]

Oh, I suddenly realize that the input custom-notsigned-prcomment is what I want!