Create separate python libraries for the following logic and refactor SBOM...

Create separate python libraries for the following logic and refactor SBOM generation script accordingly.

1) writer classes of generating SBOM in different SPDX formats
2) data classes to model the SBOM structure in SPDX

Bug: 272358880
Test: CIs
Test: build/soong/tests/sbom_test.sh
Test: atest --host sbom_writers_test

Change-Id: I1175cf0d99864bc4304559a59484ef0ba401cd64