Switches to nwa formatter for SPDX headers

[codefromthecrypt]

This removes license headers from incidental files and tightens the license header format to match other tetrate projects (which also match recent norms in the industry).

This uses a 2 line license header as there is nothing notable in the LICENSE file, and also copyright year is always wrong, so removed it.

// Copyright func-e contributors
// SPDX-License-Identifier: Apache-2.0

To apply this, I chose to use nwa, which itself is pure go, actively maintained, not single-person dependency, and apache licensed.

While is a new project, but solves the problem and also is used by a close connection beeai.