metadata: Add license-mapping.conf for SPDX license mapping

Add a file defining mapping of SPDX-2.0 license identifiers to Gentoo
ebuild licenses.  This was originally discussed on Gentoo Bugzilla [1],
the submitted to the gentoo-dev ml [2].

The proposed file uses the common key-value format used for
configuration files.  Currently only `spdx-to-ebuild` section is defined
but other kinds of mapping can be added in the future.  As described
in the top comment, it provides the ability to map SPDX-2.0 license
identifiers along with the `+` and `WITH` operators to arbitrary
Gentoo LICENSE strings (including all-of and any-of groups).

The initial list was based on cargo-ebuild's license.rs [3],
courtesy of Leonardo H. Neumann and Georgy Yakovlev.  I have done some
changes, particularly removed some incorrect mappings and added more
variants from the same license groups (e.g. GPL variants).  Ulrich
Müller has noticed a number of problems and given helpful suggestions.

A parser for this file format is implemented in pycargoebuild [4].

[1] https://bugs.gentoo.org/706472
[2] https://archives.gentoo.org/gentoo-dev/message/01924bb71411696dce04445d85da508d
[3] https://gitweb.gentoo.org/proj/cargo-ebuild.git/tree/src/license.rs?id=b4314edc5bb672f5e94dcbc996f443e98fdb4dba
[4] https://github.com/projg2/pycargoebuild/

Closes: https://github.com/gentoo/gentoo/pull/28256
Signed-off-by: Michał Górny <mgorny@gentoo.org>

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