Add global schema to tenant schema conversion and upload scripts

[ejsuncy]

#2060

Do Not Merge.

This is a convenience branch that adds 2 scripts for converting paid media global schemas, fieldgroups, and datatypes to tenant-based ones. This allows us to convert and upload proposed global schemas as tenant-based schemas to an org and sandbox to validate the schema format before the proposed schema PRs get reviewed/merged.

This branch should be rebased each time the following stacked PRs are updated with schema changes:

#2061
#2063
#2064

Usage:

1. Check out this branch
2. From the repository root, run the following to create tenant-based equivalent of the paid media datatypes, fieldgroups, and schemas. It will output those files to [output_dir] (which is tenant-schemas by default).

./convert-xdm-to-tenant.sh <TENANT_ID> [output_dir]

3. Change to that output_dir, and from the [output_dir], run the following to upload the tenant schemas to the specified org and sandbox using API credentials that have access and permissions on the sandbox:

cd tenant-schemas
../upload-tenant-schemas.sh