feat: add `libraries_bom_version` in metadata (#1956)

[gcf-owl-bot[bot]]

• feat: add libraries_bom_version

• add unit tests

• remove isfile

• add tempdir

• restore

• add current dir

• restore template

• remove unit test

• restore templates Source-Link: https://github.com/googleapis/synthtool/commit/63cc541da2c45fcfca2136c43e638da1fbae174d Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:dc11e2119c854b57b7f7ed9962391decf002454700020a52b5d0d46c40b80a7e