Add a ruletype that checks for the presence of a file header

[rdimitrov]

The following PR adds a ruletype that checks for the presence of a file header.

It accepts a filter input which is a regex for filtering out files and also the content of the header to check for. Both are required fields.

Example usages:

...
  - type: file_header
    name: enforce-license-header-yml
    def:
      filter: "^.*\\.yml$"
      header: |
        # SPDX-FileCopyrightText: Copyright 2024 The Minder Authors
        # SPDX-License-Identifier: Apache-2.0
  - type: file_header
    name: enforce-license-header-license
    def:
      filter: "LICENSE"
      header: |
        # SPDX-FileCopyrightText: Copyright 2024 The Minder Authors
        # SPDX-License-Identifier: Apache-2.0
...

[evankanderson]

A few minor questions, but this seems handy. It would be doubly-handy on PRs, but we can add that later.

[evankanderson]

Do you want to support date-matching here, or keep this exact match?

[evankanderson]

I don't think you want a security advisory for this.

It does seem worthwhile to think about what a remediation rule for this would look like, in the context of broader remediations, but this example seems a bit too limited on its own to recommend a remediation.