Add SPDX-FileCopyrightText and SPDX-FileType to source file header

[bact]

Context

In FOSDEM 2025 SBOM side event and someone complain about source-file-level SBOM information
(in general for the whole industry, not particularly this library).

What this PR does?

• Put copyright text to SPDX-FileCopyrightText
• Add to source files SPDX-FileType: SOURCE
• Add missing copyright header - using this template Copyright (c) YYYY Source Auditor Inc. where YYYY is the first year that that file appears on the GitHub repo
• Add few missing Javadoc

Question

Do we like to keep the full license text inside the source file header? Or it can be omitted? (we already have SPDX-License-Identifier: Apache-2.0 in the source file header and have the full license text in ./LICENSE file)

• if it can be omitted, I will push another commit to remove it

See also: spdx/spdx-java-core#28

[goneall]

LGTM - I also updated my IDE copyright template to use these fields