Add license and SPDX identifiers

[leha-bot]

We will use the Boost Software License 1.0.
Also the Blue Oak License is very permissive and suits for our Open Source library.
Also we must add SPDX-Identifier to all source file.
For Boost Software License it will be:

// SPDX-License-Identifier: BSL-1.0

For Blue Oak License it would be:

// SPDX-License-Identifier: BlueOak-1.0.0

The SPDX identifiers also support the logical expressions (like OR, AND, etc.)
The combined SPDX-License-Identifier would be:

// SPDX-License-Identifier: BSL-1.0+ OR BlueOak-1.0.0+

See slso https://spdx.dev/ids/ for reference.

Also note that the next commits fixing the missing identifiers should refer to this issue.