Support offline validation (no network) using only built-in, local schema

[mrutkows]

Resolves #113

[mrutkows]

Note: The following CycloneDX JSON schemas use imported ($ref) schemas:

grep -inr "\$ref\": \"[a-z]" --include=*.schema.json | sort | more
./bom-1.2-strict.schema.json:602:          "$ref": "spdx.schema.json",
./bom-1.2.schema.json:584:          "$ref": "spdx.schema.json",
./bom-1.3-strict.schema.json:570:          "$ref": "spdx.schema.json",
./bom-1.3.schema.json:553:          "$ref": "spdx.schema.json",
./bom-1.4.schema.json:1692:      "$ref": "jsf-0.82.schema.json#/definitions/signature",
./bom-1.4.schema.json:614:          "$ref": "spdx.schema.json",
./bom-1.5.schema.json:3794:      "$ref": "jsf-0.82.schema.json#/definitions/signature",
./bom-1.5.schema.json:748:          "$ref": "spdx.schema.json",
./bom-1.6.schema.json:1253:          "$ref": "spdx.schema.json",
./bom-1.6.schema.json:5040:      "$ref": "jsf-0.82.schema.json#/definitions/signature",
./bom-1.7.schema.json:1253:          "$ref": "spdx.schema.json",
./bom-1.7.schema.json:5040:      "$ref": "jsf-0.82.schema.json#/definitions/signature",